Use K2 v2.10.0 (dev) as the final release is imminent: github.com/getk2/k2/archive/master.zip

The only in-between changes relate to minor changes in the frontend templates. Just compare each template override package referenced here getk2.org/documentation/tutorials/77-k2-template-override-package-comparison to spot any changes you might need to migrate over to your K2 template overrides (if you have any of course).

Upgrading K2 to the latest release (even dev releases) is always 100% safe.