Reply to S4 software to T2 upgrade fee?

Selling me S4 but I'm not sure if can be arsed to go thru the ball ache of unregistering my free free copy of T2 just to buy another copy straight away. The soon to be owner of the S4 is happy to just have the S4 software if as long as the upgrade to T2 won't be the full normal price.

Any one know what the score is, do NI subsidize a paid upgrade from the S4 software to T2? Or am I gonna have to bite the bullet on this one