A few comments. First, any upgrade depends on the ticket in use. Some tickets are marked right on the Complimentary, or Non-Upgradable. In which case you obviously cannot add a 6th day to them. The same will also usually apply if you tickets with a date bomb on them. Often tickets sold to groups holding conferences or to sports teams through WWoS say Valid from XX/XX/XX through XX/XX/XX (typically 14 days in either direction of the event) are also normally non upgradable. Any normal ticket that doesn't fall into these case should be upgradable.
Current policy to the best of my knowledge, is that within 14 days of first use, if a ticket still can be used for park admission, you can upgrade it. So If you use day one on January 1st, you would have 14 days (an expiring ticket) to use the other 4 days. Now if you use them all in a row, so Jan 1, 2, 3, 4, 5 you would need to upgrade on one of those days prior to park close. You cannot show up on Jan 6 and look to have days added. If you use it Jan 1, 3, 5, 7, 9, you would need to upgrade by park close on Jan 9. This helps to stop scalpers and resellers from buying otherwise empty tickets once a guest is done and adding more days on at the cheaper price and then reselling for full price.
If you are getting a discount on the face value of the ticket, or are using a ticket you bought earlier buy never used for admission, enter the park at least once prior to doing the upgrade. By entering the park at least once you can request that the CM "bridge" the ticket, and you would get the value of the gate price applied to the new ticket. If you do it outside the park before you enter, you would get the price that Disney received for the ticket from the reseller (which would be less than you paid for it). So if you paid $200 for a $225 ticket, but it was bought for $175 from Disney. If you enter the park with it, the ticket is now worth $225 towards an upgrade. But it you upgrade before entering, they will only give you $175 towards and upgrade.
