Sure, changes can be made for a fee. ($100). A Thank you ticket is just like a ticket you buy at the lowest class available at the time. This means no blackout dates, as long as you advance purchase 14 days in advance I think.

Tickets are nonrefundable but you can always cancel them and use them towards another. It actually might make sense to use 35k miles for a ticket to HI when they're pricing out at 1200 or so a ticket, cancel it, then use that towards other things, generating a value of 3.5c per point, which is 7c per $ spent,not bad.