I learned recently that the company I work for would like to move me to a project that might have some technology overlap with my side project. I'm happy to move to the project, I just don't want to have any problems in the future. How do people usually handle that with employers?
At the very least you should inform the company about the upcoming conflict of interests on the horizon. Let them know you won't be able to start work on that project until you've both figured out your legal ground.
As common advice goes, seek a lawyer's advice. Alternatively talk to the company lawyer whose job may also involve sorting out things like this so you can save on fees.
Be aware that the company lawyer represents the company's interests, not yours. Any advice they give you will be slanted from the point of view of what's best for the company. (Any reasonably ethical attorney will also disclose this conflict of interest, and many may refuse to provide legal advice in this situation for this reason.)
As common advice goes, seek a lawyer's advice. Alternatively talk to the company lawyer whose job may also involve sorting out things like this so you can save on fees.