You can only upgrade a ticket to an Annual Pass while the ticket is still valid. So, if you have a one-day hopper you must upgrade on the one day that you use your ticket.
If you have a two (or more) day hopper you can upgrade on the last day you use your ticket. Most multiday tickets are good for 13 days from first use. So you can go once and wait 10 days (or 13, however long until it expires) to go your second day, and on your second day you can upgrade to an Annual Pass. You must upgrade on a day that your ticket is still valid and able to get you into the park. The expiration date of your annual pass will be the date of the FIRST day you used your park ticket.