I'm a Windows user and I love kdenlive so much that I was willing to use it on a slow Linux VM (before the Windows implementation).
I always try to use the latest version, not because I want the latest and greatest, but I just want to be able to give feedback and support the open source project.

Since I had had issue with a new version for few times and no longer able to open new project in old kdenlive I started hesitating to move to new version. I know I can report issue and many people are willing to help. But usually I'm in the situation that I just want to get the project done first and later try to simplify my project to report an issue.

My question, how can I edit the new XML and let the older kdenlive open it?

The good old days when I could open v18 project in v17
Now I'm still using v21.04.2

Just shouting something Already tried it? To see what's happening? Partly load? Nothing load?
If nothing ... what do you think the problem is? Why it doesn't load?
Warning. Never on the original. Make a copy for testing first.

Optional:
You could also export the clean tracks (whithout effects) to import in the older version. Is quicker then starting all over.
Or even export the tracks with effects if there good, for further editing.

For a long-time running 'project' i often use (parts of) tracks from previous edits in new ones. Edited or non-edited. Workes great.