If a coupler is too high you can place a shim between the coupler pocket and the car floor. If it's too low, you can either cut away the under frame or add washers between the truck bolster and the car body. You also must make sure the trip pin height is correct. If the trip pin hangs too low it will sag in your track work, and if its too high it won't uncouple. Converting your pike from one type of coupler to another is very difficult.
Many manufacturers sell conversion kits and provide instructions to help.
Make sure when you're converting that you make a few cars that have an old end on one side and a new end on the other. This way you will be able to use both new and old couplers.