Seems also that since Ruby is not going to be taught as part of people's normal formal education in programming, you can expect Rubyists to be on average more capable of... learning new things.
So yes, "re-train". Give everyone a book on the new language, maybe pay for some online courses from pluralsight or wherever, cancel meetings for a week. You can learn a lot faster than in a school environment when you've got paid 8 hour days to put into a single subject + coworkers to chat with.
Besides, it's not like they don't get to avoid learning new things anyway, even if you restrict it to the Ruby ecosystem. In the JS world (which I'm sure they all know too, as one tends to when working on web sites even if you're mostly back-end) as new revisions of the language come out people have to keep up with the syntax and changing idioms.
"For some reason, programmers love to learn new stuff, as long as it's
not syntax."
-- Steve Yegge
