Read the two sections on 'buying...' and 'selling...' so you understand what to look for as a buyer, and what the seller needs to do: https://www.propellerheads.se/suppor...ion=buyandsell
Reason licenses work in a 'stack'. The first version you buy, you buy a full version, and you get a 'base license'. When you purchase upgrades, you get an upgrade license, which is tied to, or stacked on top of, your base license.
When you sell reason, you have to sell your whole stack. You can't just sell your base license, or an upgrade - so as a buyer, you need to receive the whole stack.
If the seller has registered their licenses, they need to initiate a transfer to you, at which point, props will transfer the entire stack.
if the seller did not register their license, they need to send you the registration card(s).
technically, if the seller sold you R2.5 and had upgraded that to R6, they also sold you R6 (and any/all versions in between).
If you have the registration card and disks for R2.5, send a photo/copy/scan of the disk, registration card, and proof of payment to props support along with any contact information you have for the seller.
Per props resell/transfer policy, if you show proof of purchase and possession of media/registration, they should resolve the transaction in your favor, including giving you the seller's R6 license.
Basically, though, you took the right approach in that you can purchase any older version and upgrade - just know that the seller has to sell you their entire set/stack of licenses/upgrades.
It's been posted here many times... many sellers think that they can sell their older version of reason now that they are no longer using it because they bought an upgrade - and the stories are almost always that the transaction resolved in favor of the buyer.
and if it does resolve in your favor, you should end up with the R6 license the seller purchased - and could technically sell the upgrade you purchased.