You can mount the 2015up vantage seat to a pre 2015. If you are using it in island mode stability is not an issue, and its nice sitting higher up, makes for easier hiking out and drier ride.. You just need a set of the new rail blaza rails, I used the 21in length(could probably get away with the 9in).
I like the exra length, as the rail give you a better sense of security and more area to mount things. Just mount the rails along the seat well, put some rubber on the deck where the rear of the seat will sit. Than all that is needed is to run a strap from rail to rail for the front of the seat to sit on. Using a cam buckle strap will allow for hieght adjustment in the front.
If I want to go in kayak mode i just remove the seat and strap and drop in the original.
When mounting the rails be sure to use a good backing plate. Only 4 holes required to mount the 2 rails.
Also make sure you leash the seat incase of capsize. Its an expensive seat to donate to davey jones's locker.