I second this idea, if it’s feasible. As noted elsewhere in this thread, the lead-acid batteries in UPS units have a limited lifespan, even if not regularly drained. Solar and off-grid enthusiasts have determined that parity between overall lifetime cost of lead-acid versus lithium batteries was reached years ago, and now it’s firmly in lithium’s favor, mostly due to the greater number of recharge cycles.
Contraindications for lithium batteries would include:
high local costs for lithium battery packs
lack of space for the hybrid inverter, as they’re usually not rack-mountable
the homelab drops below 0 C (32 F), in the specific case of LiFePO4 cells
That said, breathing life into old equipment is usually more environmentally friendly than acquiring new equipment.