Replaced my LT245/75R16 DynaPro A/S tires after seven years (OEM with van purchase via SMB Fresno with a 60K mileage limit).
The tire owner's manual said to change them out after six years if you haven't worn out the tread by then. The tread was still almost like new - right up to Lincoln's nose. However, there were a few serious small cracks in the tread on at least two of the tires and one looked nearer to a puncture it went so deep. So, better safe than sorry after only 30K miles.
I had Sears install four LT245/75R16 DynaPro AT-M tires for $688. Consumer Report's 2016 magazine rated these pretty high (just 2-wheeling, but need the M+S certification for my area). Various auto web sites gave them pretty good reviews too. The ride is much smoother than on the A/S tires and it's a tad bit quieter too, both of which make me very happy.
We'll have to see how they wear but it's likely that I'll hit the time limit again before hitting the 50K mileage limit on them. If so I will switch one out for my OEM spare which would be 13 years old by then. That's about as much risk as I can stand.