Hi Natip, I am assuming the Virtual disk that you are modifying is not the one with the OS? If so yes you can use OMSA to delete the existing virtual disk and create the RAID 5. 
Hi, Couple virtual disks. I need to reconfigure one off the virtual disk from raid10 to raid5. Could I use OMSA, delete that virtual  drive and use Virtual Disk Wizard to create new virtual disk with 8 hard disks? thanks 
Virtual disk reconfiguration