What is the best way to go from high gloss white to flat black?. How do I prep the board? Light sanding followed by some sort of deglosser or other chemical?

You would probably be fine with just putting a couple of coats of the flat black on there.

You can use a primer designed for darker colors. It looks grey and its thick. I used it in the theater works well.

I'd sand....or wire brush the details in the trim. It'll take 15 minutes, and youll never have another chance to do it.
You want the black to bond well so as to reduce any chipping away of the black showing white !

I was more worried about the gloss not allowing the paint to stick and thus chipping easy. Shouldn't the gloss be removed?

Sand with a sanding sponge if surface is uneven. I would then paint it with a good oil base sealer then paint it black. I made the mistake of painting over a semi-gloss in my bedroom moldings and I can scrape the paint off with my fingernail:mad:

I agree with other posters......you need to sand, it will "rough" up the surface and give the paint something to bond with. Primer is a good idea as well. If you are doing this on baseboards and will be using a vacuum, you don't want paint chipping off each time you bump something against it. I would use either a semi-gloss or at least satin finish rather than flat. Any mark or finger print will be visable on flat.

Cheers,
Jay