I think, since you appear to have easy access from below, I'd cut the riser, replace the subfloor, then use PVC, a no-hub connector, and a pvc flange with a SS ring on it to attach to the new finished floor. You may need to go to a plumbing store to get the proper no-hub, since copper OD is smaller than PVC, and, they'll likely have the pvc flange with SS ring. The big-box stores usually only carry them with a painted steel ring, which can rust to nothing if there are leaks. The no-hub connectors they typically carry are for pvc-pvc, so they have the same OD, you need one with different diameters on each end.
The hardest part of this is cutting the copper so it is straight across, then getting the burrs off since you probably don't want to buy a pipe cutter for that size.
The proper place for the flange is on top of the finished floor.