InDesign 22.x (2026) file format questions

InDesign 22.x (for 2026) seems to have a different file format than 21.x. At least, when I tried a pre-release version of 22 and opened a file, edited slightly, and tried to save, it would not Save, it would only Save As, which is a pretty good indication that it uses a new file format. Is this true? If so, can an older InDesign such as 21.x open this file format if one needs to revert to an earlier version (e.g., 21.x) of InDesign?

On a related note, I found that editing a big file in a pre-release version of InDesign 22.x was way, way faster than editing the same file in my existing version 21! With big files in 21.x, I often use Copy Editor (Beta) because in 21.x I sometimes have to wait a second for each character I typed to appear on the screen. But pre-release 22.x was as fast editing a very big file as 21.x was with a very small file. Or even faster. Very excited and can't wait for the official 22.x release!