Funny timing. I received a couple of Matchbox cars yesterday and thought about this very question. Apparently people have been doing both - stripping and overpainting. I decided to try stripping, because the original paint is a bit thick and glossy. It would take a strong spray undercoat to make new acrylics stick, and the idea of tree heavy layers didn't sound good for details.
There are many kinds of prepaints, varying greatly between product lines. However, your usual stripping methods should work. Gentle chemicals for plastics, anything for metals. Brake fluid, pine oil, acetone, oven cleaner, true paint removers...all fine for their usual range of materials. Disassembly may be helpful. I took a test car and applied some Fairy Power Spray. A while later the paint was coming off nicely from the metal parts. Plastics weren't affected so quickly, but I believe those can be overpainted quite easily. Their old paint wasn't as thick as in the metal parts.
I can post a final report a bit later.