I have seen flight attendants decline the old drink coupons on more than one occasion. They make pretty firm announcements that the old coupons without an expiration date are no longer valid. YMMV.
I had around 115-120 of those old coupons and bought the entire plane drinks (as far as the coupons went) about a month before they changed the policy. The lead FA was willing to use them and even made an announcement about it. Three or four passengers took the time to say thank you to me as they got off the plane.