Honestly? We don't know. 2e started out as clarification to 1e. You can pick up a 1e module and play it pretty easily in 2e. And you could pick up a BECMI module and play it in either 1e and 2e. But 2e wound up being a major change to the game, and being quite divisive in its time.
3.0, 3.5, PF1 are all similar to each other as well and you could mix together most of them with each other. But again, there's lines drawn - sometimes deep, bitter lines between each of those rulesets.
In all the above cases, players were strongly encouraged to "convert" their characters over to the 'new' system. Once converted over, the problem of editions falls squarely in the lap of the DM.
I feel that similar rules, which allow a DM (with a moderate amount of work) to run a 5e module in 5.5e is probably the absolute best case scenario anyone can hope for.
I also feel that whatever happens, it's going to split the community. This situation is more than passingly similar to the 1e-2e, 3e-3.5e situation. You have a large percentage of the community who feel that 5e is the only edition, because its the only edition they've played (for years). Any significant change to that edition is going to cheese some people off.
Giving them two plus years to fret over it isn't going to help matters either. We're a screwy species. Under the best cases, a whole bunch of us will automatically look for the worst case outcome. Some are very vocal against any change whatsoever. If WotC only loses 10-15% of their player base from this, they will be extremely fortunate. Depending on what they change, it could be a lot worse.
And I don't buy into the idea that they will "listen to the people". Any meaningful change will be baked into the new rules. You can do all the internal/limited ext. playtesting you want beforehand, but its only when you release it to the world do you really know whether you've screwed up. And at that point, you already have the next couple years' worth of products planned and partly written. You are locked into those rules.
I hope everyone had a restful summer, because the next three years are going to be a vitriolic mess.