No wrong - you get the entire value of the ticket you were holding but the AP start date is backdated to when you first use it.
And when you bought it does NOT dictate the credit they give for the upgrade. As long as you use the ticket at least once for park entry, its value is the CURRENT gate price of that ticket on the day you are doing the upgrade. But not really relevant since you can only upgrade within the first 14 days of using it, so a years-old partially used ticket can't be upgraded anyway.
No you cannot do this. Tickets can only be modified or upgraded within 14 days of their first use - even nonexpiring tickets. The tickets you are describing cannot be upgraded in anyway - only used as is. By your first post, I thought you had an unused ticket, not a partially used ticket.