As others have said, I am going to call it what it is: 5.5e. A revision, which will replace things
officially (read: for organized play), but which will be effectively backwards-compatible with pretty much everything that came before. There's little to no problem playing 3e content in 3.5e stuff, and adventures written for either one will require little if any change regardless of the rules you choose to use. Much of it is simply rejiggering the math to patch up the holes they've found (e.g., giving the 5.5e Warlock a 1/day recharge of their spell slots because people don't short rest often enough.) There are some winners and some losers, some cool things and some annoyances. So it goes.
They're just bending over backwards to prevent the kind of backlash that happened with the last time they revised an edition rather than coming out with a new one. I find this pretense rather annoying and would prefer that they just be honest with people. Of course, the cynic in my soul recognizes that such honesty would backfire, but its optimist counterpart questions exactly how inevitable that is, given how much the D&D fanbase has changed over the past 20 years.
For real though:
- It's changing a bunch of spells, feats, and a handful of common/general mechanics (e.g. weapons), but nothing ground-breaking
- It's altering the mechanics of several core classes to improve them
- It's rewriting some subclasses to improve them (read: to make them suck less in most cases, e.g. Berserker, Champion, and Beast Master)
- It's going to change presentation and style, but not much about the math itself, other than a few back-end tweaks
- Most content from before will be usable without any adaptation. A few things, like backgrounds, will need adjustment.
This is exactly what 3.5e was. A revision that preserved the overall rules structure, but made tweaks and changes within it to fix what things could be fixed. It
is 5.5e. But calling it that is too much of a risk, so we're going to get silly naming conventions instead.