Yeah, but the problem is that the batteries are still full of juice.

Obviously for rack based UPSs you'd "just" take out the UPS, or battery drawer, and replace somewhere more safe, or better yet, swap out the entire thing.

For more centralised UPSs that gets more difficult. The shitty old large UPSs were a bunch of cells bolted to a bus bar, and then onto the switchgear/concentraitor.

for Lithium, I would hope its proper electrical connectors, but you can never really tell.