I would be more concerned with the charging circuitry in the UPS. They are designed for use with the batteries they install at the factory, not large capacity batteries. You may run into problems with it not being able to detect battery capacity and as an extreme you may unduly stress the charging circuit with the longer charge cycles needed for the big batteries.
Assuming it's a current-limited "float charger," which is I think what almost all UPSes have (they just float some voltage, like 13.8V, over the batteries with some current limiting circuitry so they don't charge them too fast), how would you do that?
If anything, by using larger batteries for the same length of outage that you'd use small ones for, the batteries wouldn't be run down as far. That means while charging that they wouldn't draw as much current initially (although they'd draw it for longer), which might make them easier for the UPS to charge.
What I'd imagine you might want to do is charge the batteries initially with a fast/slow charger, and then hook them up to the UPS to keep them topped off.
