I’m not sure it would have to, actually. Take the rogue. They currently get their subclass at 3rd level. If 1D&D rogues get their subclass at 1st level, and thereafter gain subclass features at all the same levels 2014 rogues do, then structurally all the 2014 rogue subclasses still would still essentially work for 1D&D rogues. They would only be missing a 1st level subclass feature. Just throw in a catch-all like “if your subclass doesn’t grant a feature at 1st level, you can take a 1st level Feat instead” or something. I dunno, just spitballing.