late in your trip. From the ticket sticky above:
Any MYW ticket can be upgraded within 14 days of first use. There are no exceptions, other than those discussed in the next paragraph. This applies even if the ticket has the No Expire Option. If the first day of use is a Sunday, the last time an upgrade or modification may be done is 11:59:59pm of the second Saturday.
In addition, effective August 1, 2011, if you have used all of your entitlements on a ticket the last day of use of entitlements is also the last day that you can do any sort of upgrade. So if you had a four day ticket and your fourth day of use was a Friday, from Saturday onward you cannot do any upgrade even though it is within the 14 day period. Note this does not apply to to tickets on the KTTW or to multi-day Florida Resident Tickets; in these cases the Guest name is preprinted on the ticket and they may add days after the last entitlement is used as long as they have a photo ID that matches the name preprinted on the ticket.
Note that you can add days or WPF&M to any ticket that has all the major theme park days used, provided it meets the requirements of the previous paragraph.. However, you can not exceed ten days of total major park entitlements.
It will be cheaper to add an additional day. An additional day will be about $10 plus tax per person. A one day ticket is around $90. I suggest you use your ticket for at least one entry before adding the day. If you have a discounted ticket this will bring the value of your ticket to the base rate. You could even use the Guest Relations location inside the first park you enter as you go in, or better, as you leave. Touring first thing in the morning is the most efficient because of shorter lines.