Normally the RAID array config is stored on both the controller memory and on the disks. I do not know if Dell does this too ...
Lets pretend they do this to, then ...
If you replaced the motherboard with the eaxct same model (with the same RAID controller) there should be no problem. Just change the settings in BIOS so that the controller acts as a RAID controller.
I think the warning is just standard way of telling that JBOD SCSI drivs content will be gone if you set the controller as RAID. But please, PROCEED ONLY AT YOUR OWN RISK. If you are unsure, call Dell Support and ask them.
But anyway i think (not 100% sure) that after setting the controller to RAID functionality, it will recognize the existing array !
But, please have a backup just in case.