On another thread you said you bought 5 day MYW tickets thinking you could use them to go to a second park on 1 or 2 days so I assume you are thinking of adding No Expiration because of the unused 1 or 2 days.
Adding No Expiration isn't practical if you only have a day or two left because No Expiration is charged according to the total number of days on the ticket and not on the number of days left. Disney World tickets are front loaded meaning longer tickets cost less per day. The difference in cost for a 3 day base ticket ($291.81 inc tax) and the 5 day ticket ($323.76) is $31.95, but adding No Expiration would cost $170.40.
If you have partial arrival and departure days you could go into a park for a couple of hours since you'll have the extra days. Otherwise, it would be better to eat the $31.95 for the unused days than pay another $170.
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