Negative, reinstall is required.BTW if your using the on board raid that came with your motherboard it's probably a software raid. So if it seems a little slower it may actually be slower because it's not a hardware raid.I discovered that after the fact with my Motherboard.
No, this can not be done since a RAID set up has to combine the drives as one. RAID 0 boosts the speed because it breaks every file up into smaller pieces and stores them on both drives. Since both drives operate at the same speed and at the same time, it requires less time to access the information since it is accessing a smaller amount of information on each drive. The drives need to be wiped clean in order to configure the RAID properly.