This would allow us to reshape the compressed research tree without breaking the predefined hardcoded tiers (= columns).
If you think it's necessary to visualize tiers throughout the whole game, add the digit next to Size in the top right corner of the popup.
Personally, I'm much more interested in a research tree that shows projects based on their size aka cost of research points, rather than tiers that are only needed to decide which techs are known at game start.
For backward compatibility you have to know how many bytes you use for the project definitions - I guess you know
Old save, double column to tier, done. And this would only be necessary if you need the tiers after game start for any reason. Otherwise column would be column and save wouldn't even become larger (one, two, four bytes per project, depending on your data structure) at all.