RE: 3 questions...

quoted

tell kernel which devices to use for your root filesystem (if your 
root is on raid device), by using something like
  append="md=2,/dev/hda1,/dev/hdb1"

I have some partitions with the fd type in raid arrays but 
would like to switch to the manual method you just described. 
These partitions hold data I care about. Is it safe to change 
the partition type:
   - while the array is not active?
   - while the array is active?

I would recommend to umount the array and stop it if you can.  The fd or default 83 (IIRC), is just a label on the disk, and won't destroy data.  I've more experience with labels on Solaris, and have done those "on-the-fly" with no affects though.

Remember (with RedHat Advanced Server anyway), you'd have to actually disable autodetection.  I'm still working on how it works because I'm still unclear whether this is a kernel feature, or just a result of init scripts.  An older RedHat reference:
http://www.redhat.com/support/resources/tips/raid/RAID-4.html

Marco