I believe the only reason the minimum changed was perhaps because the 75psi was a little low, and some were not paying close enough attention to the boost chamber pressure when they opened up the volume and then ran the shock. If you had 75psi in your boost chamber at minimum volume adjustment, you could open up the chamber and pressure would obviously drop below 75psi. That seems obvious, but people not as aware of all the tuning componenets on these shocks could easily do this and then have air migrate to the damper oil. Since, if I remember correctly, the DHX 4 doesn't have volume adjust, one can't make this mistake. The wide tuning parameters on these shock is great IMO, since rider requirements, preferences and the differing suspension designs can alter the demands on a shock a good deal.
I think fsrxc is right on the years. My '05 and '06 DHX models were 75-200.