Hi,
maybe this will work for you:
connect to Host A with your vSphere Client
click file - export - export ovf template
... export the ovf file to a datastore of your choice
then connect to Host B with your vSphere Client
click file - deploy ovf template
follow the wizzard to import your vm
This should help you to migrate the vm to your 5.0 Host.
Regards
Andi