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"a spare HHKB I had lying around" - if only we could all be in that situation 
I love my HHKB, but I have a feeling that a 55g version would be even nicer. I cannot, however, justify doing something like this to my one and only Topre board.
I own both a Realforce 55g and a HHKB. I can't fathom taking either apart, I'm too afraid I would beef it and end up breaking a $200 piece of electronics.
The RF 55g is amazing but I'll just "deal" with the "bad" layout.
Exactly!
And what would you do with the RF afterwards? Put the 45g domes into it and make a semi-variable RF?
That's exactly what I did. My brother users the RF now, he doesn't seem to notice. Especially since its an upgrade from a keyboard he got at the dollar store (EZ Eyes, if you want to look it up). I tried my best to place the 45g domes in areas where they wont effect general typing, but of course, there aren't that many extra keys on the RF.
I can recommend doing this mod rowdy, it really is very simple. I only used one screw driver for the whole thing, and it took me an hour. I can give you more details if needed.