What are the real question a here? Based on the Playtest so far... Can you play a 2014 and 2024 built character at the same table? Very likely with less "power" difference than a champion fighter and a min-max sorcadin at the same table. Can you mix and match 2014 and 2024 rules? Only by applying the sidebar. If you choose a race from 2014 you gain no ASI from Background. Can you use either in a previous adventure? Sure, and the monster encounter rules will be just as loose as they were with 2014 characters. Let's be real it's not like it was hard to build a group of characters that would run rough-shod over those adventures. They weren't built for power gamers but meant for more casual DMs (like me), that didn't give two hoots about "balance". Lastly is it hard to use Prof Bonus per Day along with Short Rest subclasses, well maybe annoying but totally doable. Will this change invalidate the core 2014, only if you choose to let it.