From the Mousesavers website:
"No Expiration Option - Regular Magic Your Way Tickets expire 14 days after their first use. Choosing the No Expiration option means you are free to come back and take advantage of unused days at any time. The No Expiration Option can be added after the initial ticket purchase, but you'll have to pay the full upgrade based on the original length of the ticket. For instance, if you buy a 7-day ticket and decide on the 5th day of your vacation that you won't be using the rest of the ticket on this trip, you can add No Expiration for $65 plus tax and use the remaining 2 days on a future trip. Such upgrades are allowed within the first 14 days (day of first use plus 13 days). Thanks to Rick G for confirming this."
It's also possible to add upgrade to a Hopper ticket, but again the price will be based on the initial ticket purchase.
See Mousesavers Disneyworld Ticket Page
"No Expiration Option - Regular Magic Your Way Tickets expire 14 days after their first use. Choosing the No Expiration option means you are free to come back and take advantage of unused days at any time. The No Expiration Option can be added after the initial ticket purchase, but you'll have to pay the full upgrade based on the original length of the ticket. For instance, if you buy a 7-day ticket and decide on the 5th day of your vacation that you won't be using the rest of the ticket on this trip, you can add No Expiration for $65 plus tax and use the remaining 2 days on a future trip. Such upgrades are allowed within the first 14 days (day of first use plus 13 days). Thanks to Rick G for confirming this."
It's also possible to add upgrade to a Hopper ticket, but again the price will be based on the initial ticket purchase.
See Mousesavers Disneyworld Ticket Page