Welcome back, Maggie! I agree with Steph - a class on fanfiction sounds amazing!
I have definitely had those thoughts when looking back at some of my old stories, but I leave them up in their original state so I can see the improvement I've made since writing them. Like Steph said, it's a good way to see growth.
I have only tried rewriting a story once, and it was a terrible experience. I got burnt out, and I don't think the story really got all that much better. It just ended up killing any inspiration I had to keep writing it, and so neither version got finished.
Instead of rewriting old stories, I tend to recycle certain aspects of their storylines in new stories and improving on them. It's fun to see how much better I can tackle certain topics now than I could then, and doing it with different characters and storylines keeps it fresh.
If you are going to rewrite, my advice would be to keep a copy of the original versions of your stories somewhere, even if they're not posted online. If nothing else, you'll enjoy going back to compare if and when you do finish the rewrites.