In 2008, I was booked on a R/T on the Empire Builder when my outbound trip was canceled due to an in-progress blizzard. I was given the option to re-book both my outbound and return trips at no extra cost. Instead, I canceled my outbound trip (and got a full refund), flew to my destination two days later, and pushed back my return trip on Amtrak by two days. Amtrak did not charge me extra for changing my return trip, even though it wasn't specifically affected by the blizzard.
That said, that was nearly a decade ago and policies may be different now. Also, I did not attempt to rebook the trip several months in the future...the agent thought it was a reasonable request and made the change. (It probably freed up space for other delayed travelers, as it was during the Christmas travel period).