I have only ever tried to upgrade a hopper to a hopper, just increasing the number of days. I have done it several times and always been charged the difference between the pre-purchased 5 day hopper price and the value of my 3 day hopper. So if they are not discounted tickets it would be $335 USD - $295 USD to upgrade a 3 day adult park hopper to a 5 day adult park hopper which would be $40 USD. I have also upgraded a 5 day park hopper ticket to an annual pass. Again Disney used the dollar value of my park hopper and applied it to the cost of an annual pass and I paid the difference. I am pretty confident that they would allow you to upgrade a non parkhopper to a park hopper, again using the dollar value that shows on their computer system when they scan the ticket at the ticket booth. I am not so confident that you could upgrade a child ticket to an adult ticket as I have never tried this. Although I have been with someone that was allowed to use the value of their expired ticket towards the price of a new ticket. The rule of thumb seems to be that whatever value shows on the Disney computer system is the amount that can be applied to the new ticket.
I did call Disney directly this afternoon about upgrading the Canadian Resident Discounted ticket. The CM told me I "should" be able to add days to my 3 day hoppers but may have to pay the difference between the ticket I have and a 5 day undiscounted parkhopper which according to my calculations would be $113.75 each. He said they would try to do the best "proration" - his word

at the ticket booth so they may only charge the difference between the two Canadian discounted tickets. It was not a definite answer so I guess I will just have to try it when I get there and hope for the best.