Confirming you have reviewed the appropriate section in R80.20 install and upgrade guide. there is section specific to upgrade of VSX, etc. link HERE>
I suggest mgmt "upgrade" to always include "migrate export <>" with re-image (especially if you running virtual which makes this wonderfully easy with good fallback options). In the case of R80.20 and SmartCenter and/or SmartEvent, you'll want to be running XFS filesystem on end result so re-image required.
the following may come in handy as well depending on your backend storage architecture (single disk, RAID, SAN, etc).
Managing partition sizes via LVM manager on Gaia OS
http://supportcontent.checkpoint.com/solutions?id=sk95566
How to extend partition on Gaia
http://supportcontent.checkpoint.com/solutions?id=sk114115
How to resize XFS partitions in R80.20
http://supportcontent.checkpoint.com/solutions?id=sk131332