Hi Downshore and welcome to the forum,
Very sorry to inform you but once you have owned a power boat
your brain is permanently damaged and you will never be able to learn how to sail.
Only kidding of course. A background in power boating
gives you a huge head
start since you have already learned many of the skills needed to operate any kind of boat, sail or power. Things like navigation
and repair of boat systems and engines (if you get a cat you'll have two engine
to work on) and general seamanship.
To learn the specifics of sailing. I could take you out in an afternoon and, assuming you are reasonably intelligent, teach the basics of sailing; how to hoist and trim the sails
and go upwind, downwind and tack. Then you can spend the rest of your life fine tuning those skills and expanding on them.
But back to your question, the best way to learn. That may be up to you. Some people learn best in a structured environment
, others do quite well self taught but at some point, however you do it, get on a boat with someone that know the ropes and go sailing. Could be in a formal school
setting, could be chartering a boat with an experienced friend or a professional captain
Initially I learned most of the basics on my own, reading and studying everything I could get may hands on, interspersed with occasional cruises with experienced sailors. Of course reading all the old discussions on Cruisers Forum can't hurt.