i'm not certain i understand the resizing of the cuff bracelet
process you are describing, however it sounds like you'll have to
take the cab out of the bezel first.
If you can avoid removing the backplate and simply resize the
bracelet,saw it and file and finish to size it.
OR- apply a heat shield ( vigor brand works best, second to wet
wadded newspaper, or any number of other products from kool jool to
rio's soldering clay) to the cab and with the most minimal flame
necessary, remelt the solder & remove the backing.then construct
another setting,solder it to the cuff and reset the cab..glass that
is hot and dry cracks, at best, and explodes at worst. If you keeep
it protected you may get away with not removing it if you work
quickly and/or used med. or easy solder to begin with..If you used
hard plumb solder, i think removing the cab is the answer.