with only two drives in the array you, of course, cannot migrate to RAID5. Have you added the new drives to the existing array and then tried the migration?
See
https://community.hpe.com/t5/ProLiant-Servers-ML-DL-SL/Convert-RAID-1-to-RAID-5/td-p/6569804
and
Page 77 https://support.hpe.com/hpsc/doc/public/display?docId=c03909334