This topic is relevant for me right now, as I am also building a custom save file for myself, redoing all the premades and lots to my specifications. It's a lot of fun, but also, it's taking forever!
When editing the premades, I make it a rule not to change their faces, with the exception of hair and makeup and accessories. I don't change their traits, as I have read removing traits doesn't always work, and the Sim can still get moodlets from removed traits even after they are gone. That being said, I do have a mod that allows me to give extra traits to each Sim, so I can make their personalities more to my liking. I've been playing the Sims franchise for 22 years, so I have preconceived ideas about a lot of the recurring premades. For premades that have appeared in previous games, I use the Sims Wiki to look up traits and other info, and kind of blend them together to make their latest version in the Sims 4. Sometimes there are inconsistencies, with things like eye colour or hair colour in previous games.
I've always really enjoyed playing premades (they are like old friends at this point) and then I add in some extra characters of my own into the mix.
I've been working on the Goth / Crumplebottom / Bachelor family for several weeks now because I want the premade Sims 4 Goth family to have all their extended relatives!
I wish you luck editing your save file - it's a big job, but I know it will be worth it in the end!
