that is a curious point out, i had to find the scene, you're right... that is so curious and subtle though... considering all the things the write staff like to make blatantly clear, this is so subtle and vague, that it's easily missed the first time around, if not almost missed.
the thing i don't care for, is the high level of speculation the plot causes in the first movie, and it doesn't help that the comics don't give any insight to Sunset Shimmer's in depth character, neither does this movie, we are completely blind to any sort of depth, and even in Rainbow Rocks doesn't completely make up for the way she was presented in past media.
don't get me wrong, i want to like Sunset Shimmer as a character, as a past villain even, but she was presented in such a way that it makes it hard for me to take her character seriously. it's cinema sins like this, makes me want to fire the entire staff of MLP and kick Hasbro in the face. i've already testified that i liked the movie, but i refuse to let speculation let me like her character, because plot holes are plot holes, and i've given plenty of anime the same treatment (especially Clannad and Angel Beats!), but i still like those stories as well, but by far none of these stories are perfect at all with the way they do plot and characters.
i honestly wish there was a sufficient canon backstory for Sunset Shimmer, instead the team presented her as nothing more than a selfish bitch, that wanted to conquer the world of Equestria with mind controlled teenage humans? are you serious? how dumb of an evil villain is she? the mane six fought off Discord, Sauron, i mean, Sombra, The Changlings, and even beyond all doubt somehow mcguffin deus ex machina Tirek, an impossible enemy... granted, i think a couple of those situations came after Sunset Shimmer... but even so, she couldn't have been that foolish to think she could overpower the mane six, Luna, Celestia, and Discord if he felt like helping... not at least with mere human mind controlled zombies
i digress, why should i merely complain? no one merely likes complaining, let me tell you what i would have headcanoned...
let's say Sunset Shimmer was kinda like Twilight, not really interested in friends at first, maybe there was a reason, maybe Sunset was a nerd growing up, had issues talking to other people because she was too interested into her own obsessions (hence, Aspergers, and since i have Aspergers, i am allowed to talk about it, lol), i don't care if she actually has Aspergers or not, just nevermind, continuing on...
let's say Sunset got picked on as a young filly, and growing up she just alienated others with her own interests, instead of working at bridging gaps, the gaps merely got wider as she wanted to earn love and respect, but eventually gets deluded into thinking, the only way to be respected is to be like Celestia, a powerful ruler with great magic, but she missed all the finer importance of what makes Celestia a great ruler, which is mutual respect and love for others.
Sunset Shimmer grows more and more hungry for power, and like in the comics, becomes banished by Celestia to no longer be her student. Sunset parts ways for many years (but does not go into the crystal portal, i will explain why later), and eventually comes back to the castle to fake apologize to Celestia, and promised to turn a new leaf, but this is a facade, Sunset instead had devised a plan to steal Twilight's crown, but needed an excuse to get close enough to her, and she knew she wouldn't have been welcome unless she acted like she had become repentant.
any sense of urgency to have guards watch her activities at night then vanish, and Sunset takes advantage of this and sneaks into Twilight's room, steals the crown, accidentally bumps into something, wakes them, they chase her, and then as Sunset gets around a corner, just as you would think she would try to teleport away, a guard bumps into her and she drops the crown, and panics, has to use her magic to levitate and is too stressed to teleport, finds her way into the crystal portal room, notices a warning sign stating that it leads to another world, and decided to chance it and enter inside.
i know what you're all going to say "but Sunset Shimmer was supposed to be in that world for longer so she could establish herself there", i think it would be more entertaining to see Sunset struggle with walk and using fingers, but even more so showing her develop proper use of such at a slightly better pace than Twilight. now this is going to sound ambitious, but for the sake of a 2 hour movie, i want to show Sunset shimmer's manipulative side at work, and have her cause rifts between the humans, instead of it just being stories told to Twilight.
also, we need a scene with Pinkie Pie rocking out to Andrew WK's Party Party Party! just saying...
Snips and Snails are easy to impress pushovers, Sunset could earn their allegiance within a day, how do i know this? (Boast Busters, 'nuff said), honestly, the only thing that needs to be different is that Sunset can't be a full on jerk, like how the Harpies actually have some depth, i think Sunset as a villain needs some hint of vulnerability shown, and once in a while needs to be shown being nice, even if it's to use someone.
then before getting towards the end where the omega boss battle happens, have her alone and nearly crying, monologue about how when she finally gets the crown, people will love and respect her finally, as she wishes to conquer Equestria in her rule, not to specifically be an evil tyrant, but just to have people bow down and marvel at her power. i think the whole "evil, tyrant, conquerer" paint of her villain motives was too simple and bland, but if we can paint her as just as a misguided and desperate for love, as corkscrewed as her vision is, i think would make for a better and more appreciated complex villain.
but i will admit, that crying during the transformation, that is worth keeping, but a couple things i have to add... Sunset is not retarded, i do not want to treat her as a fool, YOU CAN NOT TAKE OVER EQUESTRIA WITH AN ARMY OF TEENAGE HIGH SCHOOLERS. instead, i am going to drag this movie out, because come on, we get a quick deus ex machina fight like with Nightmare Moon, how lame for a feature length movie! instead, i will make Sunset shimmer teleport away, while her zombies try to fight the heroes, and something, a song sings, something, the heroes undo the zombie enchantment on the students, love, friendship, etc.
meanwhile.. Sunset Shimmer encounters the human armies of the world while searching to build up her forces, and while Twilight and company try to find and stop her, she amasses millions of special ops and Marines and MI6, you name it! imagine Sunset Shimmer with enough German tanks and Japanese mech suits to even make Celestia flinch! and then we'll have Sabaton work with Daniel Ingram for a song about amassing an army fit for an Equestrian world war.
okay, now we can have our moral-inducing-deus-ex-machina where Twilight stops Sunset (yeah, i know, building such a big army to not use it is a let down, but hey, if we can get some Sabaton music before this, i personally think that is good enough, it would please me at least, lol). and then have a Mr. T pony come out of nowhere saying "I PITY THE FOOL WHO DON'T BELIEVE FRIENDSHIP IS MAGIC!"... okay, maybe not, but it would funny, lol xD
speaking of Deus Ex Machina, the whole Pinkie Pie scene where she basically shotgun messages the exact reason that Twilight was in their world, that... is... a stretch, but it's amusing, because it's Pinkie, and as cheap as that was to move along plot, i am on the fence about getting rid of that moment, because even though it breaks the seriousness of the story, it's just hilarious that Pinkie is such a wonderful plot device in herself, so i can go either way with that scene.
otherwise, at the top of my head without rewatching the movie, yeah, this is what i would do to change it, otherwise keep it more or less the same other than my edits and suggestions.
anyone still reading? lol... hopefully someone likes my ideas.