But somebody obviously put a lot of work in it, so I can imagine the person working on it wouldn't be happy with a bit of pruning.
Don't worry about replacing some existing part of FreeOrion with something better. We're trying to make a game here, not establish ownership and get territorial about specific subsections of the code or content. If the tech tree needs pruning or complete replacement, it needs to be replaced.
Further, adding a new tech tree and associated content doesn't delete the old stuff. If someone wants to use the old stuff for some reason, they can still get it.
I'll go look around the forum and the wiki for some higher-level plan.
There isn't one... Which is a big part of why the content hasn't been replaced. I've been pondering ways to make such a plan for a while, but I'm busy doing other things so haven't actually done so.
How stable is the game framework? i.e. are data formats changing all the time and that sort of thing.
Some data formats change occasionally, and new data formats get added. Generally the changes are incremental, though, so if a big set of content is written, it's mostly unchanged when the expected data format changes.
Not much use working this stuff if it has to be redone every release
If we think like that, we'll never get any content written. It's important to have something to work with as we go forward, even if it gets replaced a few years later (like the current content should be). Any existing content is also generally the basis from which future content rewrites are done, and guide art creation, and also affect code changes due to routine testing and issues that arise as a result.