IF it is the day of check in, you MAY drop the first and last days. If you want to go to the park that day, but not the day you check out, you can drop the last day. If you do not want to go the park that day but you do want to go to the park the day you check out, YOU MUST TELL THE CM so that they can change the date on the tickets accordingly, dropping the first day and keeping the last. Likewise, you can drop your first and last day.
On the day after you check in, you must drop the first day (obviously) and can purchase the ticket through your entire stay, including the day you check out, or have the tickets end the day before you check out.
The rules for buying UPHs do not vary by resort.
I hope the resorts aren't giving out too much conflicting info!