HP does not support any PC that was made before August 2013 with W10 drivers.
So, in the event something doesn't work after the upgrade, you would have to check the individual hardware component manufacturer's websites to see if they have W10 drivers for any device(s) not working.
You do not have to have any prerequisite updates to install W10. If you are at W7 SP1, you are good to go.
You will probably not be able to revert to W7 using the system recovery method unless you made a set of W7 recovery disks.
If things don't work right after you upgrade to W10, you have 30 days to automatically revert to W7.