One thing that will break is the license so be ready to call support to get that fixed. I think that problem is getting fixed in 8.10 if it didn’t get fixed in 8.09.
The other thing that will break is your Edge configuration settings will get wiped out. The properties to configure an edge gateway were extended (improved) and the old configuration settings you have set up will not survive the upgrade.
My recommendation would be to screenshot your edge settings on each edge device and your main gateway so you can quickly reconfigure them after the install.