Yes. When I switched to Dvorak (college, circa 2000 or 2001) my Qwerty speed dropped from around 60 to around 20, and Dvorak, after a couple months, was up to 60. Then 120. My Qwerty speed stayed low until I got a job where I didn't have full-control of all the computers I used (shared lab systems, didn't want to be rude and create confusion by adding a layout to them even if it was opt-in). At that point I was spending about half the day on my own work computer in Dvorak (still around 120 wpm) and half in Qwerty and my Qwerty speed went back to around 60-70.

If you don't want as big a delay (several years for me from when my Qwerty speed dropped to when it recovered), I'd suggest going "all in" on Dvorak for 3-6 months and then integrate some Qwerty time in deliberately to recover and then maintain your Qwerty proficiency.

Yes, maintaining your proficient in QWERTY is only a matter of not stopping to use it altogether when switching to a new layout. It'll just be a matter of time spent to become fluent in both. I recommend Colemak however.

I personally didn't maintaing typing on QWERTY, interestingly my brain still has the QWERTY mapping for tablets.