Hi
My first Topre board, a few months ago, was a 91-UBK. Indeed, it's a little awkward at first to type on some keys you were used to type on, and just don't have the same input than before. (Sometimes it just has no input at all, that's pretty weird when you start typing on this though).
I couldn't adjust my typing to it, so I sold it back (not the primary reason, but whatever).
I guess you could use some soft to "remarry" keys to another position if you'd like to.
Using a different layout than the one you're used to is always a tad bit awkward (as when I switched from AZERTY to QWERTY, and now I'm using QWERTY on AZERTY boards), but you'll eventually get used to it.
The major problem, IMO, is that you'll have two boards, with different layouts, and that might be a few days of use to "correctly" switch from one to another.
Other than that, the mini-spacebar we find on Jap-layouts Topre boards is great actually, and it wasn't weird at all for me.
I hope you get to use it well with your adjustments, I think that's a good idea if you've already got a spare set