Not completely sure what you are asking. But in a tail dragger aircraft the CG is behind the main land gear (MLG), while a tricycle configuration places the CG in front of the MLG. You achieve this by placement of the MLG, tail draggers generally have their MLG mounted near or forward of the lending edge of the wing, while tricycles' MLG are placed toward the middle or aft part of the wing.
Of course moving the landing gear around will move the empty CG of the aircraft and a tricycle configuration will generally weigh more.
There are many kit planes that allow you to build the aircraft either way, but I don't know any that would allow an easy conversion from one to the other.