I would do the following, assuming you have a good backup capability:
Step one: Make a complete backup of the C:, D:, and E: array, (and system state) and verify the tape can be restored from. If possible, store a copy on the other array to restore from faster. This is your safety line if all else fails.
Step two: Run DCPROMO on the server in question to demote it from the DC role (not necessary, but it will greatly simplify the process). Now make a backup of the old C: drive in the non-DC (member server) configuration, including the system state info. Saving a copy of this to the other array will speed up the recovery also.
Step three: Run your SCSI configuration utility and re-configure the first array to the configuration you want, then run the installation manager to install the original OS back onto the new C: drive in Array one.
Step four: Once the OS has been installed, run restore and recover the original OS configuration (of the non-DC server, restore the system state, then restore all the data etc., for D: and E: (from the first set of backups) to the other partitions of the array.
Step five, run DCPROMO and bring the new configuration back on line as a DC.
You system should now be back to the original configuration, but on larger partitions. The idea if 15 GB for C: is nice, and I would not go smaller than 10 GB at least, depending on the amount of RAM you are using.
HTH,
David