You could stick the toilet in the middle of the room, it can stand alone anywhere...might not look good, so moving a 12" toilet design and installing it on a 14" rough-in would be fine. Some wouldn't like the tank away from the wall, but it would work fine.
To transition from copper to PVC, you'd need to go to a plumbing supply house for the proper nohub connector. The big box stores tend to only carry those designed for pvc-pvc, or pvc-CI. The ID of both the 3" copper and the 3" pvc are the same, but the ODs are different. You need the proper nohub that takes the OD size difference into account. If you get that, there should be no 'step' in the pipe when properly joined.
Extending the pipe nearly 2' means nearly another 1/2" of rise. PVC fittings are often taller than copper ones, and depending on where the pipe is now, that extra rise may make it hard to get the required fittings in there.