I think I have a plan for this. I have some rubber feed pans like this
They are made of heavy rubber that is flexible and soft. If I can place one of these over the main drain and get some weight on it or hold it down with a pole, it should restrict enough water that the hole won't fill on me while I cut the old fittings and install the new.

I have fittings to work both compression and glued, depending on how things work out.

I also am in the process of modifying a standard expanding rubber plug by welding a piece of conduit on so I can slip a ball valve on after the plug is in place. I'll post a picture when I get it made and can try it out on a practice pipe.

Last piece of the puzzle is finding a reasonably priced full flow PVC ball valve to connect to the pipe and still allow me to withdraw the expanding plug after the valve is installed.

I'n not going to do the work until the pool is ready to open in the spring.