I'm running out of ideas... About the only things I can think of now, that I remember people suggesting is:

-Feed a live prey, and after they've swallowed it, and still somewhat in feed mode, offer a f/t. Apparently, this is supposed to get them used to eating f/t and after a few weeks they should be able to make the switch. But since it sounds like your BRB is still small, it might be too much food, unless you try fuzzies.

-I've also seen people suggest dipping the f/t in tuna juice...

I haven't personally tried either of those two methods, so I can't vouch for their validity. Some snakes just refuse to switch. I hope you find something that works...