Straps to strapless bridal alteration HELP!

I bought my dress from Pronovias and it currently has straps. It looks very couture with the straps but I think i've decided that I want to do a sweetheart strapless. The Pronovias seamstress said she would be able to do it but i've read reviews online for the Pronovias alteration department and people don't seem impressed by them (it's also $700!) so i'm looking for recommendations from someone who had their dress completely redesigned. Not just normal alterations but something huge done to their dress. Thank you so much! I have 2 months left so I gotta get moving!