Hey Eddy,
Though this is generally safe, best practice is always to migrate the VM's to the other host prior to the upgrade. If there is a shared datastore, do a SVmotion so no VM's are on the host.
If VM's are on local datastore, take a complete back up of your VM's before upgrade or cold migrate them off, as one needs to just forget to choose "keep my vmfs volumes intact" once and you can run into issues.
Regards
a
