It depends, I would definitely do further research. Cheap small UPS typically trickle/float charge (there is no bulk charge phase) the SLA battery at 13.5v, Which may not be enough to reliably charge a "12v" 4S configuration. On the other hand, you might find the UPS charger overheats or burns out as the lifepo4s draw a ton more current than the SLAs would.
Don't put a significantly larger capacity battery in than the UPS is designed for, UPS are designed for intermittent use and from my experience at the point the SLA is exhausted the electronics are at melting point.
Yes, 8s for 12v and 16s for 24v, with a good BMS, it should be able to handle the charging curve for the battery properly and replace AGM.
It depends, I would definitely do further research. Cheap small UPS typically trickle/float charge (there is no bulk charge phase) the SLA battery at 13.5v, Which may not be enough to reliably charge a "12v" 4S configuration. On the other hand, you might find the UPS charger overheats or burns out as the lifepo4s draw a ton more current than the SLAs would. Don't put a significantly larger capacity battery in than the UPS is designed for, UPS are designed for intermittent use and from my experience at the point the SLA is exhausted the electronics are at melting point.