I'm doing a bathroom gut and have run into an issue that I've never tackled before. I've stripped the floor of its linoleum and 1/4" wonder board. I noticed that the 3/4" subfloor also needs to be replaced so I'm going to move forward with that. Here is my problem while I have the floor and piping accessible...

I'm going to change the layout of the bathroom which will require the toilet flange to be extended 19" from its current location. This extension will take place along the same floor joist (accessible from the basement). No 45's or 90's just a straight run. The run is about 12 feet of 3" copper and is properly slopped at 1/4" for every 4 feet.

I'm thinking of a copper to PVC union once the pipe is cut then attaching the closet flange once I install the new 3/4".

For you plumbing pros...How would you extend it? Are there any issues with a waste line if you have a union from copper to PVC? Meaning, will anything from that connection potentially "catch" solid waste since it's no longer a seamless run? Also, the new flange will be a 14" rough-in from the wall due to baseboard heaters, any issue with that other than the extra 2" behind the toilet? Thanks.