When I get a pre-qualification, am I stuck using that lender, or can I switch?

You can switch lenders at any time you would like. You will just be going thru the process again with each lender.

Realistically, you can switch anytime you want in the initial shopping and pre-approval stage. But once the loan rate is locked, and you are physically moving forward on an actual house, with a purchase agreement, etc. It generally isn't wise to change.