Can I convert a virtual IDE disk to SCSI disk?
Yes, you can. SCSI disks perform ~20% faster under Virtual Server.
Download the 8 page Microsoft whitepaper called “Virtual Server 2005 – IDE to SCSI Virtual Machine Migration” from Nelson Araujo.
Note:
There is also some more info in Kris Bennett’s post “Converting an virtual IDE drive to SCSI in Virtual Server“
If the disk was bootable you may have issues with boot.ini, see John Howard’s post “Boost Virtual Server Guest Performance“