A huge issue concerning newcomers to Colemak is that they think their QWERTY skills will fall off a cliff and that they'll be doomed to typing at ridiculously slow speeds on any public computer for the rest of their life once they learn it. This thread was inspired by my own concerns this would happen when learning Colemak for the first time, then realising it totally didn't happen. Currently I'm just as fast as I was before on QWERTY, and 20 30 WPM faster on Colemak and way more comfy.
HOW CAN SOMEONE WHO TYPES ON COLEMAK SWITCH BACK TO QWERTY WITH FEW ISSUES?
If you're like most people, you did not learn proper touch typing technique when learning to type on a computer. This is what it's supposed to look like:
With the circled keys representing where your fingers 'naturally' rest. You've likely heard that this is how it's supposed to be, but you don't do it yourself. Instead, you might 'hunt and peck' with two or three fingers maximum, with little regard to how much your wrists ultimately move (loads!). If you have never tried to type correctly, this is the category you fall into.
WHAT DOES THIS MEAN FOR ME?
If you did not learn proper touch typing (like most people), it is highly likely you will be able to switch between Colemak and QWERTY.
Whenever you want to go back to typing on QWERTY (because you're forced to by some nasty public computers), just start using the wrong technique again and you'll find that once you get used to it you can switch between layouts with ease.
Colemak is a layout built with touch-typing in mind, i.e fingers in the 'correct' positions. When learning Colemak it's highly important to learn it with proper touch-typing technique! Because you didn't learn proper touch typing with QWERTY however, your brain has separated the two layouts in your head based on the typing technique you use..
When you are learning Colemak, it is normal to see your QWERTY speed drastically decrease as your Colemak speed increases. However, after you have mastered the Colemak layout, your QWERTY speed will likely come back to nearly all of what it was before. This is because your brain no longer is in cognitive overload from consciously thinking about where all the keys are. You probably won't be as good as if your main layout was QWERTY all the time like before, but typing speed is likely to be comparable and very good for normal typing.
REASONS THIS MIGHT NOT WORK FOR YOU
If you used proper touch-typing technique on QWERTY, switching between layouts might be a lot more of a challenge because your two layouts use the same typing technique. If you are older, or do not have much experience in learning new motor skills (video games, speedcubing, sports involving your hands) for instance then I think you will find this more challenging as well.
IS THIS A HARD AND FAST RULE
No. There are some proficient touch typists in both layouts. And some people will forget how to use QWERTY even if they did not touch type with it. The human brain is really complicated and I no idea how to explain those discrepancies. However, your former technique on QWERTY seems to be a good indicator in explaining how well you can handle two layouts. This post is based on theory, not evidence, definitely take it or leave it.
BONUS ROUND
Once you get amazing at switching between layouts you HAVE to try Layout Swapper, a game that may drive you bonkers as you franctically switch between QWERTY and Colemak trying to type words before you lose. Give it a shot!