[erksa]

It's my own experience, and it anecdotally seemed to be shared with other people who have modified their main keyboard:

The regular qwerty/qwertz keyboards are so ingrained in our muscle memory and society (think touchscreen keyboards) that they still work as backups and you do not "forget" how to efficiently use regular keyboards. I guess it does depend on how often you're swapping around.

[bluGill]

I started with Dvorak young - about the same time I was learning "proper" typing (as opposed to hunt and peck). I wasn't good enough at QWERTY to go back and so I eventually had to give up. Besides, all the studies of typing I've seen are not compelling - most don't try to control for that fact the alternative layout users are likely to study typing and thus be better than average.