Is this the drain pan inside the indoor air handler or a galvanized pan under the indoor air handler. If its the former then they are probably correct; if the latter, then of course it can be changed.

Sounds like a furdown ac unit.

Your tub in this case acts as the emergency drain pan, that's why they placed them above the tub.

Typically these were installed in condos, but some cheaper built homes got them as well. The newer furdown's typically don't last as long usually due to Freon leaks. Most units now come with plastic condensate pans. So they rarely fail via the drain pan any more.

Typically when I see these in a house, I recommend that the furdown be absconded, the HVAC system be relocated / new duct work installed. If the home is a condo or town home, you might not have a way out other than to go back with another furdown unit.

If this unit is R22 Freon unit, you're in a tough spot. R410a is the replacement refrigerant, but in a couple more years... this refrigerant too will begin being phased out. (Kigali amendment to the Montreal protocol likely to be fast tracked now with Dems in control.)

So when the new furdown unit starts to leak, probably within 5 or 6 years you'll have the joy of replacing the equipment again for the newer refrigerant (whatever they decide that to be)

Yeah alot of fun coming your way..... (I'm in the process of forming my plan with all these incoming changes)

I service the Katy, Texas area.