If its virtual make a snapshot first. Easer to roll back. But at least backup the db and keep all the modules that match the current version, so you can reinstall if it goes wrong
Consider using the backup license feature, build an exact copy, and then backup you primary to the backup. You can test the upgrade on that, and then run from the backup while you upgrade the primary.
Check that there are not any 7.1 or earlier modules if you had upgraded previously. Ones that were obsolete are not always deleted. You will get an error if any 7.1 modules are still there.
I had issue with WSM database, WIP. It would not upgrade. I had ot undeploy the module and it will prompt to delete that db. This allowed it to install a fresh copy. As it had no data for me no loss.
Anything you "fixed" like a custom image, backup script or panel or something not working quite right in 7.2, you should keep a copy. It may get overwritten with the default stuff.
Allow 2-4 hours for the whole process at least - its slow, as you also have the patch releases for some modules.
Have a list of critical things to test so you can verify that the features you need still work as intended - some things will break, either new bugs or bad data from previous not converting properly.
You may have to redo some things becuase they comsetically have moved - topoplogy stuff for instance.