In my experience, the only way to make sure they don't lose their NPC role is to edit them while you're in live mode. I edit gym trainers all the time and they always keep their job. What I do is, while I am playing my Sim, I find the NPC at their job location. Then I open the cheat console and type in testingcheats true hit enter and then cas.fulleditmode and hit enter and close the console. Then I shift-click the Sim and click on edit in CAS (or something similar). Then I edit them as usual but before I leave CAS, I make sure that I have selected the outfit category they were wearing when I entered CAS. Otherwise - although I don't think this happens with homeless households - it can happen that you switch over to that household when you exit CAS and are playing the Sim you only wanted to edit. And this is how I edit 90% of game-generated Sims, not from Manage Households but on the go.
Librarians are a bit of a different creature though because they wear a career outfit that you can't edit in regular CAS. There are separate cheats for that which I have never tried. But you can edit the physique and features this way of course.
