As IB issued the ticket, you'll have to deal with IB. Their ability to rebook you will depend on award availability on the flights you want. If there is none, you may be able to request IB to contact their CX liason, but there's probably no guarantee.
Assuming this is just a single-segment round-trip, you should get a refund back for the difference in miles, of course again IB would need to handle that.