A while back I was at a FMCA area rally and got a great chance to have a long and very interesting discussion with Roger Marble (an acknowledged tire expert). He said that without a doubt, heat is the killer of tires. Heat from the sun, heat from running under inflated and heat from the road surface....
New tires compounds are pretty well protected from UV damage, but the heat of solar radiation on black tires cannot be ignored.
So, when you have replace the tires and recorded the sidewall dates in you maintenance log, then find a truck scale and get the real weights (don't just guess) and then look up what the inflation should be for that weight.
Now, in your case, your plan is good. Just replace them now and then exhale.
Matt