robmary,
To answer your questions... The design has not changed. However, Disney's policy is that the mugs are only good for the stay when they were purchased. Before you start wondering, your question is a common one and there's no harm in asking.
It's not just "some people" that have been told. Here's the sign (taken in June 2003) that posted at the entrance to Roaring Forks. There's another sign posted over the drink dispenser reminding people that they are only good for the current stay:
"The Great Mug Debate" often erupts here and takes on the tone of a heated theological debate. Let's summarize the differing views that always form in every Mug debate thread in the last several years... Maybe we can save a lot of people a lot of typing.
1) The Official Policy Group
The official policy, anytime anyone writes WDW, is "one mug, one resort (excluding sisters), one visit". Signs are also appearing in food courts around WDW that state this. Members of this group (me included), know that in practice food court CMs rarely enforce this policy and often contradict this policy* when asked about it by guests (sometimes even when the official policy is posted on a sign only a few feet away). They feel the CMs simply have thrown in the towel and don't have the resources to police the soft drink lines. This is why you may see someone using a 7-11 "Big Gulp" cup unmolested by CMs who witness the obvious abuse.
This group adheres to... the official policy.
* Update: A recent DIS poster added a new one... A CM told her mom that ANY Disney mug was OK to use at her resort.
2) The "But the CM told me..." Group
This group was told by a CM when they purchased their mugs that the mugs were (pick one or both):
a) Good for life.
b) Good at any resort.
They feel that official policy, or not, what the CM told them is what matters. They say either WDW unfairly changed the rules on them after they bought the mugs, or they wouldn't have bought the mugs if the CM hadn't given them bad information. They generally feel like a "wronged" party in the debates when people cut-n-paste the responses from WDW, and advocate continued use of their old mugs until actively prevented from doing so by CMs. The fact that most of them have long since recouped the costs of their mugs in savings from free re-fills doesn't impact their feelings.
Whether or not the current official policy is a change from the past is another side debate. Some recall the policy has been consistent, but enforcement has steadily dropped off, others claim the original policy was "for life".
3) The "I give WDW a ton of my hard earned money" Group
This group, the smallest of the three, feels that it's no skin off of Disney's back if they reuse the mugs again, and again... policy or not. They cite things like the high prices of everything at WDW, the internet rumor that Coke gives WDW syrup for free, etc. etc. as their basis for their actions. They may also borrow mugs from friends, or buy them off of eBay, or use their Big Gulp mugs for their trips to WDW.
I will also add: As for those that think that the policy has changed in recent years and what a CM told them a couple of years ago was the policy at that time, I've been active on this board from 1999 (under another ID) and from that time forward its been consistently reported from WDW when asked, that the policy is "one mug, one resort, one visit."