Before replacing the bath tub, I want to install a new tub drain with a trip lever stopper. From the photos you can see what I have left to remove is the cast brass drain tee as it mates to the pvc house drain plumbing. From the photo of the new tee, it can be seen ~2-3 threads above the tail piece. It looks to me the old tee is simply threaded into a female plastic coupling that terminates the house plumbing. IOW, the tailpiece does not join the house plumbing with a compression nut & plastic washer. Bottom line, I tried unscrewing the old cast tee, but it did not budge. I'm really reluctant to really "get after it" for fear of cracking the pvc house plumbing. The downside to leaving the old cast tee is the crud inside the tee that will inhibit the trip lever plunger. Yes, I can clean out the inside of the old tee.
Before I get myself into trouble, I would like some advice as to leaving the old tee in place versus removing it. If removing the old one, any advice to get it apart without destroying the pvc.