Welcome to the club! I think there are many teachers who are in the same boat as you. My philosophy on "re-using old projects" for the AP exam is to proceed with EXTREME caution. For students who are set-on doing that, I make sure to re-iterate that everything they write about for the written portion must be done once the "12 hour create clock" starts. I have them make comments around their old code with a disclaimer that says "// this was developed in class with the help of the teacher before the start of the create".
They must write new algorithms and abstractions that meet the criteria set by the college board for their create project.
In general, I discourage students from doing this because I don't want them to get flagged for academic dishonesty. ALSO, they have 12 hours to do something in class - even something as straightforward as "design a digital scene" gets pretty close to all the criteria for the College Board. They could pretty easily do a new "design a digital scene" project and then not have to worry about the academic dishonesty piece.
Finally, a last disclaimer that once students start the 12 hours, you cannot tell them what to do for a project - teachers are in full "proctor-mode" vs. "teacher-mode". That can be really tough for me to turn off the teacher part of me, but once students have a task, they get the idea that I cannot help them.
I would highly recommend looking at the Create survival guide to use as a tool for students who need to brainstorm ideas for the create.
SO... that was a bit of a long answer. Did it help? I guarantee you are not alone so thanks for asking the question here so others can see this too!
Happy Testing Season