Considering the most popular DAWs, which do you think would be the widest gap to cover between any two of them? What would be the worst nightmare: migrating from ProTools to Reason? From Reaper to FL Studio? From Cubase to Live?

 

From Studio One to Digital Performer.
Actually, from any DAW to Digital Performer.

 

Reason or FL Studio to any standard "linear DAW" is the worst. Otherwise, none of them are all that wildly different. You could add in trackers like Renoise, something vintage-ish like Jeskola Buzz, that new Colugo Blockhead "daw", or even the Hit n' Mix RipX daw with the first group; should you consider any of them actually popular.

The easiest way to switch is to delete every other daw from your computer. Immerse yourself in it from scratch, and get a printed book if you are really going to commit to it. It's easy to back out otherwise, especially at no cost to you; so putting a little money into it can give you some more resolve. If you are talking about migration from one to the other, that is a more detail based consideration.

 

Something really easy to something really hard. Like from Audacity to Pro Tools .