I learned to touch-type QWERTY in high school. 20 years later, I learned to touch-type Colemak, and eventually got up to the same speed. For a while, I couldn't really touch-type QWERTY. But gradually I regained the ability. It still helps for me to be looking at the keyboard, so that I have a different context to go with the different layout. But I am still using 10 fingers when typing QWERTY.