Don't think Priceline tkt will let you change your flight simply because it is Priceline. My friend bought a tkt from Priceline (AA), interestingly, AA won't check him in. Their reason? "You bought your tkt from Priceline, we can't check you in here."

Luckily, he didn't need to check any luggages in, but never the less, he had a nightmare of pre flight check in experience.

Funny as it seems, in an article about Priceline in Smart Money's issue (A couple of months ago), they had a similar story with some guy who can't check in because of the Priceline.
This happened a few months back, don't know if anything changed or not.

Don't want to scary you there, just keep that in mind and be mentally prepared.