You don't have to (and shouldn't) wait for it to fail, but you have to force it offline to cause the hot-spare to begin to rebuild in its place.  When you swap the drive, ALWAYS swap a hot-swappable drive "hot".  You can force offline the drive in OMSA in the OS or reboot and do it in the CTRL-R utility for the PERC.
I have a 2950 with Perc 6/i there is a 4 disk VD and one Global Hot swappable for a total of 5 drives. The Disk 0 is flashing yellow fault and says Predicted failure YES.  Is there any way to get this disk swapped or do I have to wait on failure ? I recently lost a primary Controls server due to multiple drive failures at the same time.  I ordered spare drives and have them in hand but have never replaced a drive on the running server before.
2950 Server Raid 5 Disk 0 has predicted failure 