How can I paint over a glossy surface that might contain lead paint?
Rather than just seal in paint that might be lead-based, especially as you mentioned that you're stripping off the bad coat, it might be worth finally just removing it so you don't have to go through these concerns in the future.  (especially if you don't have kids now, but think you might in the future, so you can get the dust over and done with).

New York has some recommendations for removing lead paint.  The important thing is to keep it from becoming dust and getting spread around the house, which just makes things worse.  I believe there are some chemical paint strippers that can be used with lead paint, although that website didn't mention any.  I also found mention that Massachusets used to require licensed contractors and require it to be disposed of as hazardous waste, so it might be worth checking before you attempt this.

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If you were going to attempt this without stripping it all off, I'd follow the New York guidelines (contain the area), wet sand to rough up the surface, then prime it and re-paint.

Oh ... and you can also get test kits for check for lead paint.  Odds are, being gloss and trim, it's more likely though, and even if the top coat isn't, there might be lead in an older coat.