Migrate VM and Upgrade OS

All,

I have 2 version of ESX, 3.5 and 4.0, I would like vm which on ESX 3.5 moved and use by ESX 4.0 (done by vCenter Converter),  but i can't upgrade the vm from Windows server 2003 to Windows server 2008 after it moved, can anyone who help me with that? any other way beside using the vCenter Converter?

i'm really appreciate for your help.

Thanks