My observations are: Cold turkey is the only way to go. It will take you a couple weeks to a month to regain your old speed. Then you will be faster. Your speed has nothing to do with the advantage of one layout over another. There is no speed advantage to Dvorak or Colemak because the intrinsic efficiency of one layout over another is dwarfed by the mind's ability to establish a skill in autonomic memory. The additional speed comes from the effort of re-training itself, and unlearning bad habits in the process.
Also: Don't try to retain the ability to touch-type on your existing layout. I tried this every time. I could do it, but only at the expense of speed and accuracy. When I gave it up, both improved dramatically.