The only problem I can see is if you lose or misplace them. Make a copy of the back of each ticket so you can try to get them replaced if you do. For the trip we recently returned from, we had 7-day tickets. DH had a work emergency and only used 4 of his days, so after we got back we mailed his ticket back and requested an upgrade of 3 more days and a no expiration option so he can use his ticket when we go next year.