Most of the information you're probably reading has to do with painting with latex over oil-based enamel. That's a tough one. If you're talking about painting over latex with latex enamel, there's usually not a problem, but the surface sheen can present problems. The higher the gloss, the smoother the surface and the tougher it is for the new paint to adhere. That's a particular problem with surfaces that are subject to frequent contact and cleaning, such as doors and windows.
Preparation is everything, so whatever you do you have to knock the shine off of the old paint. That can be done by sanding, wiping down the trim with liquid deglosser (or adding it to the new paint), or by using a primer designed for tough surfaces, such as Benjamin Moore's Fresh Start.
For painting question, you can do a lot worse than picking up the phone and calling one of the Sherwin Williams stores. They cater to contractors and are _far_ more knowledgeable about their products than anyone in an orange or blue apron.
R