So the goal is to free an OS.
Or more like you don't want to reinstall from scratch.
If it's a full install, you can simply reinstall Trisquel without erasing
/home (and backing it up just in case before). And then import a personal
Synaptic list.
Fully free, fully personalized, all free, and your previous data is
unchanged.
But freeing other Debian-based OS without reinstalling, I wonder if it's that
simple.
You'd have at least to modify the software repo.
Plus you have the non-free software recommendations that could be a problem.
What I'm trying to say is that I have no idea if removing non-free programs
is enough.