You can export them directly, you don't need to do all the storage stuff if you don't want to.
If you are able to shut down the vms the steps are:
Shut down vm in xencenter
Export vm to share
Import vm in xencenter (other xenserver)
Change mac address of nics for imported vm to match the original vm
Boot up and confirm it works
Delete original vm
If you are not able to shut down the vms the steps are:
Make snapshot of vm
Create vm from snapshot
Then all the steps above, using the new vm as the 'original'
The export will not send empty space, so if your vm is 250GB but only 10GB used, it will send 10GB.