I basically told Codex to use this and that local repo of Warehouse, upgrade FA to 7.2, use the shims and not make any changes to the actual usage of the font in the theme so I can check if the shim layer works. After I saw that the new icons are loading with the old code calling them (FA3 and 4) I asked it to inspect all the theme files and module and swap the code for the new formatting.
The old font files light stay for the initial testing and you will have to put the new FO files where they are expected to be (I beleive in 7.2 they must be in /webfonts folder under theme's /css folder). After that the bot will merge the big global.css file with all the glyph mappings in the FA source css (necessary if you don't want to have an additional call to another css), this adds around 11k lines of css.
The only issue I think I had was in FO -> My profile section one of the icons was not rendering properly so I had to change it to similar one in the new package.
In total, around 2 hours with all the testing