Talking to my boss about switching departments even though I am currently team lead of the other department

You're overthinking this. You're obviously valued and appreciated by your company (they wouldn't be promoting you otherwise), so just go to your bosses and say what you've said here - you just need to explain why you feel you would be more impactful in a purely UX role.

I really want to talk to my bosses about wanting to commit to UX 100%

That's the way to go

but I'm not sure how I should approach this

You want to sell your company on your proposal. That's a basic marketing job and so you should be better qualified to do so than most other people!

1. Flesh out the idea into a specific actionable plan to the extent that you can. Try to tackle some of the details too (org charts, hiring plans, budgets)
2. Present the plan and the motivation behind it
3. Honestly discuss the potential pros and cons & benefits and risks for the company. Don't tip toe around the cons. Any half way intelligent boss will bring them up anyway, so tackle them proactively.
4. If there is business case to made make a business case and show your numbers
5. Discuss a few alternatives or variations
6. When you are done ask open ended questions and listen carefully. The goal here is to flush out concerns and obstacles and the best way is to say little and listen a lot. Clarification question are fine but don't start to argue.
7. Once you have the feedback you can start working at the concerns (if there are any) and proceed from there.

Has someone been in the same or similar shoes?

Yes, of course. That's pretty normal and most (good) manager welcome this. Managers are also human beings: the more mature your proposal is, the less work it will be for the manager to implement it and the more likely they will be to agree.