It's not that uncommon for them to change costumes for different actors- recent example would be Fran Drescher in Cinderella, who got a complete costume change.
Yes but the Belle dress is and was iconic at the time... the point being made was that they allowed Braxton to show more skin, rather than the design changing altogether, a la Victoria Clark wearing a green dress in Follies, where Peters wore pink...
They definitely reverted back to the original style of the blue dress, at least, since that's what shows up in the later promo shots of Andrea McArdle and later replacements.
I wonder if the new/special costume designs were part of her contract? Since, according to Robert Roth, it took Tim Rice promising Toni a brand new song (which he apparently did after a bottle or two of wine) for her to finally sign her contract.
Yeah I agree recently in one more broadway show the costume were changed as they told it was not looking cool so there can be any reason to change costumes in shows.