The beam bushes will be fine for either beam.
If you have just paid good money to have your diff fixed get back there and complain, unless you are happy with 2 weeks use for the money ?
It is down to them to correct THEIR mistake.
I would not go to the bother of changing the beam just get your diff fixed they are the same strength in 4x4 as 2wd rear diffs.
The change though is straightforward, from 7" to 7.5" beam.
tabetha