For all other tickets (4 & 5 day PH's, 5, 6, & 7 day PHP's, etc), you must upgrade it before 7 days from first use of the ticket have elapsed in order to credit the entire price you paid for a pass to a new one. You may only upgrade it to a higher pass, not a lesser one. (Example: You can upgrade a 5 day park hopper to a 7 day park hopper plus but you can't use it to get a new 4 day park hopper). After the 7 day window has passed, you will only be able to apply the unused portion of your pass to a new one. Once your last day of admission has passed and your ticket is no longer valid, you will not be able to upgrade. So be sure to do it on or before your last ticketed park day or 7 days from first use, whichever come first!
Keep in mind, too, that if you have unused plus options remaining on any PHP you upgrade, you will lose those unused options by upgrading.