Can see plenty of possible explanations, but you don't know what happened after the Mane Event so it's hard to judge which, if any, might pop up in future.
First, it's possible she kept it as a stage name - after all, its the name she is already known under - but moved more to a rara style of performance
Second, the glitz and glam show might well still be in demand after tME, so she may have recruited another unicorn (and either new dancers, or re-employed the previous ones) to do the lighting and SFX, That doesn't mean she couldn't also be doing rara style balads as well - either as a separate set, or under a different stage name
Third, its possible svengallop recruited another impressionable young pony and set them up as the "new" Countess. After all, almost everything about a CC performance, even the mane colour, was a "svengallop production", so it's entirely possible a lot of ponies wouldn't notice the difference.
We can only hope AJ gets a letter or something from her that clarifies what has happened since tME