Are the 550 mhz and 667 logic boards interchangeable?

Howdy! Yes. The logic boards should be cross compatible. Make sure, though, that the 667Mhz logic board is definitely from an Onyx model. There is also a 667Mhz DVI logic board that will not be compatible with your Onyx PowerBook. The only consequence of making the switch is that your original system discs will not work, since they are made for your old logic board and not your new one.

Good luck!