I'm currently a frontend engineer. I told my team lead a while back that I'd like to transition to the server side, especially since our server team is woefully understaffed and our frontend team is overstaffed. He told me "Sure but you'll need to learn all that stuff on your own time". So I never did it because I'm interested in doing other things at home.
The middle ground here is that I'd try to grasp the basics on my own time, then ask the switch. I can understand your boss though: would you swap a competent employee for an incompetent one, paying the same amount? Probably not. Yes, you retain your knowledge about the front-end, but you wouldn't be using that. My advice would be to work out the basics on your time and then ask somebody in the backend team to be invo…
But this is the glass-half-empty boss. The glass-half-full boss is happy to have a competent employee who can be called upon when needed for important front-end stuff and is picking up backend work at a faster rate than the intern the team would have otherwise, because they're already an experienced engineer (with knowledge of the front-end no less!) and there is skill beyond just writing code that transfers between domains.
And that's to say nothing of swapping a competent employee who's already onboarded for an empty seat and a talent search, when the employee quits to expand their skills elsewhere.