Just a side note, a cartridge bb is not necessarily an upgrade. If you have an all Shimano 600 bike, your current loose bearing bb should be a really nice one.

In my experience rehabbing bikes, the old style bottom brackets are almost always rebuildable (maybe 1 in 40 or 50 that I find are shot), whereas probably 20% of the bikes I find with cartridge bb are shot. With a little maintenance, the older style bottom brackets can last just about forever. If never maintained, eventually you will see some spindle and cup wear. At that point, replacement is warranted.

I tend to only replace bottom brackets when I upgrade cranksets. If the upgrade crank requires a different bb, time to change. The last four I changed were due to installing hollowtech style cranks (external bearings).

You can save some weight with modern bottom brackets..