Commercial Banking Analyst to Investment Banking

I can't speak to how internal transfers work at a BB. That said, if you're open to working at a MM or LMM bank, you may have some success getting an off-cycle position as an analyst at a bank that has sudden staffing needs.

I'd start with some self-guided research and ask a couple friends in/around IB to get knowledgeable on the basics, begin to craft your story/narrative, and then start networking with the types of banks you want to target. If done well you could be top-of-mind when a hiring need arises. It's very doable. It's different for everyone, but I'd probably break out the effort as 40% developing your story/narrative (This is what makes you unique, but don't overcook it), 20% learning about the industry through reading, 20% learning about industry through networking, 20% on some sort of TTS or financial modeling course to show interviewers that you're serious about this and that you've invested in yourself to hit the ground running once on the desk. Best of luck dude.