We have installed inverters to assist with the back up time but told the customer to leave the UPS in and the inverter can then supply power to the UPS
The older UPS units cannot take lithium and it becomes an expensive exercise changing batteries due to load shedding - At least the inverter supplying power to the UPS allows for the battery life to actually get to end of life being 5 or 10 years depending on the battery.

We also find the switching time to slow on high spec PC's and servers