I've done a bit of research on migrating existing data to a raid but it almost all has to do with migrating an OS disc. I have my OS on a separate disc than my data. I want to create a RAID for the data disc but Raid isn't enabled in the BIOS. Currently my OS disc is connected to a different controller on my MB than my data disc. I want to know if I can enable Raid in the BIOS without affecting my Data or OS discs and secondly if I can then migrate my Data disc into a Raid 5 array.
Given that your OS is on a different controller what you want to do is certainly possible; changing the Intel controller to RAID should have no effect on the OS drive.
You would need to back up all the data from your data disc as the act of creating the RAID will likely destroy all existing data on the drive(s); although simply turning on the RAID in the BIOS will not affect your data, the controller built in to the motherboard is likely to be too basic to have any migration ability.
I would just back everything up to an external drive; create the array and copy everything back afterwards