I didn't go back to QWERTY until having had a couple of months of uninterrupted Colemak use, so I don't know what the effects of that would be. But yes, I can now switch between the two :) After a few minutes of 'warm up', I can type about 70 WPM in QWERTY and then go straight back to >100 WPM Colemak. Also, I always use QWERTY on my mobile phone, since 'swipe-typing' doesn't really benefit from having frequently-used letters close to each other nor from having them on the home row.
That’s encouraging to hear that you can switch between the two. Awesome!
I’m afraid to start practicing QWERTY too soon, and risk losing my progress with Colemak. Maybe I’ll attempt it in a few months.