The reticles in the ACOGs are fixed by the factory. You just zero it accordingly and use the lower stadia for the farther ranges. The ballistics tables show very little difference between the 5.56 and 7.62 trajectories out to 300-400 yds - at least not enough difference that I was worried about it. I used my 5.56 calibrated ACOG on my 7.62 with no appreciable difference.
There is a difference, however, in the eye relief of the different ACOGs. The ones designed for 7.62 have a longer eye relief than the 5.56 models. The difference is identified on Trijicon's web site.