PR #2363 contains significant changes to Leo's read/write code. See the PR for the many details.
Happily, these changes do not change Leo's file format in any significant way. If Leo could read one of your ancient .leo files before this PR, Leo should be able to read the same .leo file after the PR.
There is a tiny chance that some insignificant data might be lost from ancient .leo files (like marked nodes) but I would be amazed if anyone noticed.
Let me know if you discover anything amiss.
Edward