Judging by the playtest and comments it’s more like a 5.1 or 5.2 edition. Less changes than 3.0 to 3.5 but enough to say it’s different-ish. If you have Tasha’s and later? Not a lot of differences to how the game is now played, but if you compare straight core to core, it will be different but compatible.Speaking of bad edition conversion/compatibility advice from WotC, this might be another "my memory is terrible" moment, but it came up recently with WotC's claims that the new thing will be fully backwards compatible with current 5e.
Does anyone else remember 4e marketing claiming that it was going to be "Backwards compatible" with 3/3.5? This would have been during that window between when they announced 4e and when the playtesting started. But I'd swear I read that claim somewhere back then but then the claim disappeared as quickly as it had appeared.