VMWare describes this problem here:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1013109

To prevent this from happening set the SAN policy before the virtual hardware upgrade:
DISKPART
SAN POLICY=OnlineAll

Set it back afterwards if unsure…
DISKPART
SAN POLICY=OfflineShared

More info here:
https://social.technet.microsoft.com/Forums/windowsserver/en-US/c14bdde3-6359-463d-9932-5fe7ec72505e/how-to-change-default-san-disk-status-from-offline-to-online?forum=winserverfiles