First upgrade to v2.8.0 which is the newest one. You will then have no issue using that on Joomla 3.x. K2 2.7.0 works as well but it's a good time to update K2 as well.

If you have any issues with the Joomla 2.5 to 3.x update itself (not K2) and you wish to start fresh, you can always just install K2 on a new empty Joomla 3.x site, delete all K2 tables in the DB of the new site and import the K2 tables only from the J2.5 site into the new DB. Finally move the /media/k2/ contents (but don't move the "assets" subfolder if you still use 2.7.0 on the old site) from the old to the new site and you're ready to start fresh on Joomla with ALL your K2 content.