Here is my experience with Dvorak and AZERTY (the French variant of QWERTY). I learnt AZERTY as a kid, then learnt a French dvorak layout, and then learnt the US dvorak layout, which I now use.
I have totally forgotten the French dvorak, but not AZERTY, because I feel I haven't learned those in the same fashion. AZERTY I learnt by hunt-and-peck (I was around 50-60 WPM when I switched), the dvorak variants as touch-typing (as the key labels don't match what's written on the keyboard, so you have to do it this way). So my feeling is that if you used to hunt and peck with QWERTY, dvorak will not overwrite this; otherwise, there's a risk.
This is also what I think is the main benefit of learning an alternate keyboard layout: force you to touch-type properly. Some people manage to move on to proper touch-type on a keyboard layout they learnt by hunt-and-peck, but others don't, and the level of comfort achieved by not looking at the keyboard at all is really, really worth it. Glancing alternatively at the keyboard and screen is stressful and causes typos.
By contrast, if you can already touch-type proficiently in QWERTY (but really touch-type), then I think the benefits of learning an alternate keyboard layout are less clear. My impression is that it saves you maybe some muscle strain because your hands move a lot less, but I'm not sure of how beneficial that is.
I have totally forgotten the French dvorak, but not AZERTY, because I feel I haven't learned those in the same fashion. AZERTY I learnt by hunt-and-peck (I was around 50-60 WPM when I switched), the dvorak variants as touch-typing (as the key labels don't match what's written on the keyboard, so you have to do it this way). So my feeling is that if you used to hunt and peck with QWERTY, dvorak will not overwrite this; otherwise, there's a risk.
This is also what I think is the main benefit of learning an alternate keyboard layout: force you to touch-type properly. Some people manage to move on to proper touch-type on a keyboard layout they learnt by hunt-and-peck, but others don't, and the level of comfort achieved by not looking at the keyboard at all is really, really worth it. Glancing alternatively at the keyboard and screen is stressful and causes typos.
By contrast, if you can already touch-type proficiently in QWERTY (but really touch-type), then I think the benefits of learning an alternate keyboard layout are less clear. My impression is that it saves you maybe some muscle strain because your hands move a lot less, but I'm not sure of how beneficial that is.