I've mentioned it before, but I really like this meme as a way to kind of find places where characters are super similar to each other -- which also shows you what might be good places to push them in different directions. Do they have exactly the same nose? Give one of them a super different nose! Etc. etc.
A big hurdle for me was realising that nothing's stopping me from wildly experimenting. You don't have to Officially Decide To Change It in order to try drawing a character with a different look. I would get really hung up on things like "but I've always imagined this character being skinny, I can't imagine them being fat," ----- and the thing that I realised was that you can go ahead and draw the character really differently as an experiment, to see if you can make it work! And if it doesn't work, don't do it, but trying it out might help you find something cool.
Sometimes just by drawing a character over and over again with a different design, to try it out, I'd get a feel for this new look, and I'd start getting excited about the new design that I hadn't been sure about before. So, go ahead and try to change some things up -- you don't have to commit to a decision before you try drawing it! What if you change someone's race? What if you give one of them really distinctive facial hair and a different nose? What if you change their body type? even if it feels weird at first, and even if you ultimately don't use some of the experiments you try, it's still good practice to try drawing it and see what it looks like!