Er... well, I've just found something that I don't know about RAID! I thought I knew RAID pretty well, but this has never occured to me. I would guess that it might not work if striping is used IF you set the new chip to use a different stripe size or something. If you ensure the same stripe size, it should work.
To be safe, I would consider backing up the array to a third disk then rebuild the new array from that, much like you can do in three-disk arrays.