Changing your cache settings will not affect your arrays or data, nor is a reinstall of the OS required. Cached data is simply data written to the cache memory to be written later to the disks, reporting the write as complete - this improves a systems responsiveness and performance. Turning the cache off (or changing to write-thru) only forces the controller to commit the write to disk before reporting the operation complete and accepting the next write operation.
As long as you have a RAID controller with battery-backed cache (PERC 6/i and H700) and no RAID memory problems that have been reported, that cached/unwritten-to-disk data remains safe, as it is kept alive in the cache memory by the battery power, which usually has a 72-hour charge.
So, it is not necessary to change it while you resolve your UPS issue, but if you want to, you can change it in either the CTRL-R utility during POST/BIOS or from the OS using OpenManage Server Administrator.