So far as I know with most of the Powerbooks you're pretty much stuck swapping boards between units of *exactly* the same generation. For instance, for most of the 15" PowerBook's life it would come in two speed grades. You can swap boards between, say, the "Powerbook5,4" 1.33ghz and 1.5ghz models from 2004, but if you tried to use a board from a 2005 "Powerbook 5,6" 1.5ghz and 1.67ghz model you're going to be SOL. IE, connectors will move/be different, heat sinks will be completely differently shaped, even the case pans different enough that you're going to have a heck of a time trying to cobble together a mutant. Googling pictures of the A1138 vs. the board of its immediate A1106 predecessor shows some different screw positions and a big hole through the board on the 1138 that's not there on the 1106, so I'm willing to hazard the guess that it isn't going to happen.

There's also the question of whether an earlier logic board would detect and work with the high-res LCD...