SV to Concierge is considered an upgrade according to Celebrity category rankings. Any change of category like this would cause a reprice with prevailing rates and promo.
Generally after final payment they would allow an upgrade if the new category was more $, at least $1. Occasionally if the new category was less they would allow the change for what you paid but there would be on accommodation for the difference. But some times even that was not allowed.
Best advice I can give is to call and see what they might offer, then you can decide.