I only buy OnGossamer anymore, and I have three different Bump it Up Bras from them.  They are all the same cup size and band measurement.  I do have to say that THIS bra (the boudoir blooms bump it up), in the exact same size as my others, is different from the regular bump it up bra. It fits a bit tighter, so I have to wear it on the 2nd or 3rd clasp. It's also a double clasp, whereas the regular bump it up is just a single clasp; not that it makes a difference.

The odd tightness of this bra doesn't make a difference to me, but if you're already on the last clasp on your normal band measurement then you might not want to buy this in the same size, but size up.  Just a tip. The cups and everything else are the exact same as my other ones.  It just runs a bit tight in the chest band.