There have been some reports about confusion over tire load ratings when buying tires -- in one case, a TireRack rep wouldn't sell a set of tires.
You should be able to clear this up by explaining to the salesman that you know about this and plan to adjust your tire pressures accordingly, so that your tires' actual load ratings will remain unchanged. Here's how it works.
First, forget the "maximum" load rating printed on the sidewall, especially for OLD OEM tires, which in those days had max. inflation pressures of only about 32 to 36 psi. The reason is that no one routinely keeps their tires inflated to the sidewall-stamped maximum (unless they want to wear out the center of the tread awfully fasst), so it's really moot. People usually (street racers aside :-) keep their tires at lower pressures. And also keep in mind that modern tires have load ratings of up to 44 psi, which can compensate a great deal.
Now, back to the real world. Explain to the tire salesmen the following:
All dealers have a publication called TireGuide which contains tables for every conceivable tire size (and ply rating, etc.) showing a tire's Load Rating which varies with inflation pressure. The actual load rating of a tire being used on a car is not the Maximum amount stamped on the sidewall (which only applies at maximum pressure), but actually is the amount determined by the inflation pressure really being used. Let's say your car manufacturer calls for a certain inflation pressure (from the door jam, or in the owners manual) for a certain OEM (or optional) tire size -- that determines the actual, specific load rating that the car manufacturer deems is SAFE for your vehicle.
Now, if you change tire sizes, the load rating for that same pressure will be different, of course. In fact, lower aspect ratio tires have lower load ratings for a given pressure, so a plus one or plus two change will lower load ratings at that pressure. However, raising the pressure used will restore the new tires to the manufacturer's specified load rating.
Example: look up on the table for the OEM tire's size the manufacturer's recommended pressure, and then note what the load rating is. Next, look up on the table for your new tire size that same load rating, and see what pressure will provide that same load rating. That is the pressure you should be using with the new tires, per the car manufacturer's load rating requirements. Again, if you went "plus" something, it will require more pressure, but likely (given the higher max pressures of newer tires) there will be ample capacity for the additional pressure called for.
This should satisfy the dealer.
Hope this helped.