Will old anchor holes in a concrete slab reduce its useful life?
The concrete looks intact enough for re-use, I can't see if the anchor bolts are good or not, if they can't be reused  getting old anchors out of concrete can be tricky: you might have to move your letter box one inch to the side so that new anchors can be drilled into virgin concrete.
If you do move it don't leave the old anchors protruding they are a trip hazard, if the can't be pulled out use an angle grinder to cut them and grind them flush.