There are several restrictions to be cautious about with these. The first is, that it must be a full price ticket. You cannot buy the ticket using any discount code, AAA discount, or NARP discount. If you do buy the ticket with a discount, the agent must cancel that reservation and issue a new one, which could mean that you pay more if the bucket got kicked since you originally brought the ticket.
I'm not sure I agree with that assessment. There was much arguing back and forth within Amtrak about that issue. The official policy is that, when using the upgrade coupon, one would *never* have to pay a higher bucket fare than booked (which, in many cases, would require a manual override). It was implied that you would have to lose the discount. However, this year, the upgrade coupons have a line that reads something to the effect of (going off of memory here since I don't have a coupon handy to check) "agent must not collect additional fare from original fare paid." That line would imply that even discounts such as AAA/NARP would not be forfeited when using the coupons.
If someone has a coupon handy, they can look it up easily enough (it's in the fine print on the back), otherwise, I'll check when I actually find them, wherever they happen to be hiding at the moment.