To transform something form one coordinate system to another, create a transform object using Transform.ChangeBasis .
If you had a point in 3-D point from world x-y coordinates and you wanted to report to the user what that point was in construction plane coordinates (of the active viewport), you would use a change of basis transformation.
If you wanted to orient an object oriented from one plane to another plane, you would not use a change of basis transformation. Rather, you use a plane-to-plane transformation using Transform.PlaneToPlane . This is because you are not looking to convert from one coordinate system to another. You are simply looking orient in the same coordinate system.
Hope this helps.