Looks like a union. Your going to need some heat or penetrant on that to get it to budge. Start by soaking the connection with PB-Blaster. If that doesn't do the trick, hit with MAPP gas and then try to unscrew it.
Maybe I a missing something but why not just use a no-hub connector with an ABS stub to transition? That will allow you to keep the existing undisturbed and make for a fast fix.