Hello Richard,
I believe the best way (so far) would be to actually go to the repository that you want to rebuild and checkout to the different specific branch/tag and then trigger the rebuild with
$ pybombs -p your_prefix rebuild uhd
You can rebuild faster by adding the '-k' flag at the end of the command, so it won't delete the build directory and start over (depends on the number of API changes this might be a good or a bad idea).
If you are using GNURadio, don't forget to rebuild is as well so it picks up the right version. And, if you identify any issues with UHD, please also start a thread on the USRP-users mailing list in order to discuss them further.
Regards,
- Nicolas