But did the nimbleness allow him to switch so much, or did switching so much cause the nimbleness?
I can juggle 5 balls, and when I do, I often hear people say something like 'wow, you have such great coordination, I could never do that', but they don't realise that before I started practising with 5 balls I didn't have the coordination either! I would throw 5 balls up and 5 would hit the ground.
Practice and determination over time built up the necessary coordination and skills to allow me to do it now with relative ease.
The same principle applies to learning keyboard layouts and almost everything else.
I guess the one thing that does help is memory. I was able to ditch the printed layout after a couple of days. But even then, it was slow going for the first week or so.
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.