I just noticed this question and according to my “old information” (I do not know if they changed it or not now) that “creating” a RAID with a mix of HDD and SSD will be blocked by QTS. Not sure about “replacing”
If that still true, I think the only method is to remove both drives, use some disk clone software on your PC to clone the drive from HDD to SSD and place them back to NAS in original order.