Yes it definetly is an option to upgrade the "older" appliance to the new version, export and then import into the new appliance. The one thing you want to make sure if is that the older appliance supports v8. Typically if it a D, E or F modelyou are just fine.
A third option, which may end up being more work, but many times ends up with a better outcome is reconfiguring your policy from scratch. It will allow you to "clean up" your policy. It is also important to understand that the upgrade process has been refined a ton over the years, but you will probably end up with an "upgraded policy" that includes more rules than you had at v7. This is due to the differences between v7 and v8 and the upgrade process ensuring that the behavior of the new policy is the same as the old.
Hope this helps,
Check out this KB article for the procedure to upgrade the firewall from 7.0 to 7.0.1 and from 7.0.1 to 8.2.1 (the KB says 8.1.2; I will fix that).
How to upgrade a firewall from 7.0.0 to 7.0.1 and from 7.0.1 to 8.x
My recommendation also would be to install v8 and create the new policy by scratch.