Booking AGR points travel does not lock in the points cost if the trip is moved to a different date. AGR award travel paid with points is directly tied to the cost of the trip as if paid with money. Except when caused by cancellations or delays, re-booking to another date is repriced to the then current fare - cash or points. If the new cost is higher, you pay more (points or money). If the cost goes down, money is refunded or credited to a voucher, and the excess points are re-deposited to your account less any applicable penalty (10% for sleeper travel). You may get an agent that will waive the sleeper re-deposit penalty. If the change is due to Amtrak, then the original fare (cash or points) is honored.
The old rule of thumb that the lowest fare for a trip is when the reservation window opens at 11 months out is no longer true. The opening fare is now often set at a mid or high fare level (bucket) hoping to snag customers who want to lock-up a reservation early. As time goes by, the fare can vary, both up and down. For most routes, May and September are among the busiest travel times.