Hi, I'm a plumbing novice learning the hard way. I'm remodeling a bath and the new floor will not be as high as old mortar bed, so I need to lower the toilet flange. The existing plumbing appears to be 3" copper pipe with cast (copper, brass?) closet bend. Tried to heat the flange to remove it and best I was able to do is remove the (3-4") piece of vertical copper pipe at the solder joint where it comes up from the bend along with the flange still attached to it. What is best technique for replacing the vertical piece and the flange to connect with the existing (cast copper or brass) closet bend?

Thanks!