Hi
Not sure I understand what you are actually going to do but you do have to follow the upgrade path. So whether you are upgrading data, config or licenses it will be the same. The upgrade path goes like this:
3.5 -> 4.0(5)
4.0(5) -> 7.0(1)
7.0(1) -> 8.5.1
So if you are only upgrading the licenses and plan to just manually recreate the config in 8.5 (no migrating of data across different tools) you will need the base 3.5 license, the Upgrade license for 3.5 to 4.0, the upgrade license from 4.0 to 7.0 and the upgrade license from 7.0 to 8.5.
Or, to simplify all of this, you may be able to go to the Licensing team and have them see if they can give you a stand alone 8.5 license. They may be unwilling to do this though but if they are you would need to have all your PAK codes and Serial numbers.
Hope this helps.