Changing the rating of a show does happen quite frequently, but usually it's for specific episodes, though series-wide rating changes have occurred. For example, a show called Wizards of Waverly Place once had a TV-PG episode instead of TV-G like normal, and Breaking Bad has had both TV-14 and TV-MA episodes. As for series-wide changes, The Walking Dead was changed from TV-14 to TV-MA after the Parent Television Council complained.
However, I don't think this show needs to be TV-Y7. Sure, there are a few moments that push the limits a little for TV-Y, but most are played for laughs and I doubt they would scare many young children. The moments I'd say have the most chance of startling young children are Fluttershy "snapping" that bear's neck and Twilight going crazy. The other moments are moments that I consider to be not very frightening, or are moments that would require background knowledge to understand why they're pushing the limits, which wouldn't matter.