If the fixed cup is in good shape, you might get away with just replacing the balls. If there are 9 ball-bearings in a retainer per side, you might want to discard the retainer, and replace with 11 ball-bearings and no retainer.
Are you sure the adjustment is correct? If there's even the slightest bit of slack, all the load will be on one single ball-bearing and the wear will be accelerated. Check
www.parktool.com/repair/ for instructions on how to adjust the bottom bracket.
If you need to remove the fixed cup but you don't have the correct tool, but you have a good bench-vice (vise) and enough space, you might hold the fixed cup in the bench-vice (vise), and turn the frame, remembering of course that the BB might be threaded LH or RH depending on whether it's English (LH) or Italian/French (RH) threaded.
- Wil