I've changed held tickets many times. From my experience, the entire itinerary does not reprice, just the parts you change. You can keep the fare buckets you already have for the parts of the ticket you do not want to change. However, if the price from that fare bucket increases, you have to pay the difference.

Also, if your held ticket violates an advance purchase requirement, you can only buy the ticket as is, any changes will reprice the whole thing.

I have tested this on dummy reservations.