... does the 8890 really do hardware raid? i didn't know that. or did you mean software raid? (i don't really know; i haven't been keeping up with the latest sager stuff)
a hardware raid would be completely transparent to the system. either os would just see it as one unit. (one drive). a software raid, however, is done by the os - and of course, the windows and linux versions of raid are not compatible.
... it is extremely hard to switch raid levels on the fly. the only safe way is really to copy all the stuff somewhere else and change it.
i believe the only way it would be easily done would be to turn a raid 0 into a raid 0+1 by mirroring the entire raid 0 unit, to make a recursive raid setup. it doesn't work the other way, though - a raid 1 has all the data written to it, so you can't stripe it without moving the stuff off and re-writing it in a striped manner. and raid 4 and 5 are completely different systems with parity bits, so those need to be completely rewritten with either the parity bits striped correctly for raid 5, or a separate drive with parity bits for raid 4. i suppose you would have a little flexibility with the linear raid mode, but that's a software raid linux-only thing, and is not actually a real raid anyway.