No one can give you a definite answer to what the cost will be to upgrade those tickets. The current policy is to charge you the difference between the gate price of the ticket that you hold and the gate price of the ticket that you are upgrading to. In your case, that would be the difference between the price of a 7-day hopper and an 8-day hopper.
The problem would be that this policy can change at any time and prevent you from doing any upgrade at all. And an increase in ticket prices is not unlikely between now and February 2017. Another concern would be if Disney restructures their ticket media. When they did that in 2005, you could no longer upgrade PHs and PHPs to any kind of MYW ticket.
And as a PP pointed out, you would not be able to book FP+ in advance for 8 days of park visits if the tickets that you have only cover 7 days.