You really are supposed to purchase a new one for each stay. Many still use the old mugs for the resort though. Currently WDW isn't selling resort specific mugs, they are Year of a Million Dreams for all the hotels.
If Disney's new mugs are different from your old ones, then it would stand to reason they did it so that people would not reuse old mugs. It is a business and each year you buy a new mug for your visit...