Well I hope the EX was not an AWD...because you cannot change just one tire....You have to change all four tires to new. If you do not then you WILL damage the differential clutch packs. Any amount of wear will cause a new tire to rotate less than the tire on the other side...this will cause the differential to engage and cause premature wear on the clutches.
Not going to happen. But, I could see a dealership telling you this.
A new Hankook tire is about 10/32's of tire tread depth. After +55,000 miles on my Hankooks, I still had 4-5/32's remaining, but I wanted new rubber to get through another midwest winter. And, I've NEVER gotten more than 45,000 miles on a set of tires on any of my prior Explorers. So, I was not unwilling to buy all new tires (Bridgestone Alenza's with 11/32's). I was happy & excited to do so.
So, the point I am trying to make is that the difference with a "new" tire and an "old" tire is pretty insignificant, even after more than 55,000 miles. The smart folks in Ford's engineering group have this figured out for us.
Have you looked at the spare tire for our Explorese? It has a FAR greater size difference than the 5-6/32's I've noted above.
I DO agree with your comment; as it relates to high performance sports cars, with AWD, stability control, adjustable suspensions, low-low sidewalls and etc.