I think that the best strategy is to go cold turkey, since that will guarantee quicker learning if you have the time for it. If you would like to retain your qwerty skill all you have to do is take like 5 minutes of typing tests each day in qwerty while looking at the keyboard, and make sure that when you type in colemak you don't look at the keyboard, this will give you some kind of mode for your head to switch off.

That will be the fastest, but if you need to get work done or just will not have the time do QWERTY by day, Colemak by night. Due to the fact that Colemak is so similar to QWERTY you should have it down quickly. It would also be good for you to memorize the layout in your head at least so that you don't need to look at a diagram before your hands get used to it.

But most importantly, PRACTICE.

Regarding me, i first memorized the replaced keys (didn't take long), then went cold turkey on my usual (physically QWERTY) keyboard.
Worked out pretty well for me. I haven't bothered trying to retain my QWERTY skills, but i could probably write with about decent speed looking at the keys.

Typing of the Dead did it for me :)

No, but I did see a link to the demo in the "learn" section. A friend sent me his copy.

Reading your review now, I see that wasn't necessary.