Had you asked me a month ago, I would have said yes... ish. It looks to me like they're changing a good bit more than I had previously expected, so we're seeing something of the scale of a 3.0e -> 3.5e change, rather than a 4e -> Essentials non-change. (I should note that I generally consider the complaints about 3.0e -> 3.5e lack of compatibility to be overblown.) So I'd expect existing adventures to run fine with the new books, possibly with a small web enhancement to update some monster stat-blocks, but I wouldn't expect to run 2014-vintage characters alongside 2024-vintage ones without issue, and might find older supplements have some teething problems.
Following the OGL debacle, I think all bets are off. If they want to create a walled garden with tighter licensing, that would necessitate more extensive changes, which would lead to incompatibility. But it would also mean essentially abandoning the ongoing design work and the playtest and starting over.
Gut feeling... yes-ish. But I wouldn't be at all surprised by the opposite.