I have Started removal of the tiles and tub to replace them with a more comfortable tub and a surround. But I can not get the tub drain to unscrew from the drain system. what is the best way around this problem.

Use a special tool to unscrew the drain. I think it's called an h-wrench. I used a pair of channel locks. Same result, but the drain looked terrible afterwards. I didn't care since I installed completely new tub, drain, etc.

You can waste alot of time trying to remove frozen parts. As kmac3002 stated, it is much easier to cut and replace the drain section. If you do insist, try penetrating oil and let it soak in for a few hours. If that doesn't help you can try heating the tub drain to allow it to expand and break loose. If neither works then replace the drain assembly.