I care a lot about file compatibility. One of the first advantages of JSesh, back in the 2000's, was that it could read both Winglyph and Macscribe files with good results (along with my own Manuel de Codage dialect, for tksesh).
In theory, a file written by JSesh version n should be readable by all subsequent versions of JSesh. The reverse is not true: if I add a new feature, it won't be supported by older versions!
If you meet problems concerning JSesh files compatibility at some point, contact me (obviously, you make regular backups of your files, not just the current version, right?).