and that wasn't eiher donated or purchased through any kind of public funding or donations to a museum or similar, then you can do whatever you like with it.
It feels like the long standing discussion about whether it is OK to repaint/restyle really rare and collectable dolls!
My own feelings are that it is very sad that irreplaceable things are used or altered when there are so many amazing new designs from which people may choose, but I do also believe strongly that people and businesses have a right to do whatever they wish (as long as it is legal/honest etc of course) with things they have bought with their own money. It is a very different situation if the funding has come from fundraising, donations, state support etc intended to purchase items for conservation and preservation.
I don't know the background to the sale of this particular dress, just my general opinion here.