The replacement batteries for my APC 3000 and BPC 3000 arrived. I've put them in, that's a lot easier than extracting old bulging batteries. I powered up the UPSes, and they seem OK.
I'll leave them on test for a bit before I start using them.
These sealed lead-acid (SLA) batteries seem to have a very variable life span, anything from two years to five. I suppose it depends on the usage cycle and the temperature.