Go to Disney's siter and find the page where you purchase tickets
select quantity 1 of the ticket you have (adult or child) and the options included on it and make note of the price on the chart.
Select the options you want to add, (ater parks, no exp, etc) then look on the chart and see the new cost.
Subtract the new cost from the old cost then take that figure and add the FL tax (6 1/2%). That will be the cost to upgrade. Don't forget to use the ticket first if you bought it at a discount. This lets you use the difference between Disney's cost of the 5 day and Disney's cost of a 10 day.
If you upgrade before using one day, you have to pay the difference between what you paid and the 10 day Disney price (although I'm not sure how Disney would know how much you paid for the discount ticket...can someone explain this)