Yes, you can upgrade regular tickets to AP. How much credit depends on the ticket and when you purchased it — credit will be equal to what WDW sold that ticket for on the date you purchased. Not necessarily today’s price or the price when you do the upgrade if a price increase has occurred since you bought.
Package tickets cannot be upgraded until arrival. Stand-alone tickets can be upgraded at any point from pre-arrival until the date you use the last entitlement.
Incredi-pass will activate on first use, just tap at the tapstiles. Other APs that are restricted may need to be activated at Guest Relations or a ticket window to show ID proof.