Yes, tell the Agent you want to Modify and Upgrade your Rez, not Cancel it ! An Upgrade is done at the Current Bucket (Price) and sometimes is even Less than the Room you had! (This is rare but it does happen, before you call use amsnag.net,a handy tool developed by one of our members, to check daily prices for up to 30 days at a time! Prices can vary by hundreds of dollars on different days for the same train/Room!)
The agent should be able to quickly modify your rez and charge (or credit) your card with the difference in cost without charging you a refund fee(10% on Amtrak if booked after the deadline for refunds) which you dont want, you just want to either pay the difference or have the credit charged back to your card! If you get one of those agents that says they cant do it or tells you they have to charge you a refund fee ask for a Supervisor!