I think the change occured even before 2001. One of the big pluses to buying the mugs when they were first introduced was being able to use them "forever" - especially for those of us who tried to make at least one or two trips each year.
Since my boys - and husband - really liked the mugs, I'd always throw them in the suitcase so they would have them for each trip. But when we returned to the Contemporary 5 or 6 years ago, my husband immediately noticed it was a new mug design and the sign now said "good for the length of your stay."
We spoke with the food court manager and he agreed to honor our "refills forever" mugs - then he offered to exchange our old mugs for the new ones.
Hmmm, old mugs that are good forever or new mugs we can only use for a few days???? It wasn't a hard choice and besides, we liked the old design much better.
During the next few years we stayed at the BC and YC and did not buy their mugs because they are such a hassle to refill. Since then, we have stayed mostly at the Swan because of the great discounts and the Starwood perks.
But I'd be very interested to see what would happen if we stayed at the WL or Contemporary again and brought along our old mugs. I guess they wouldn't let us use them - even though when we bought them we were told we could "use them forever?"
Disney NJ