Remember in the old days, when we already union or subtract two 3D solid objects, then we no longer able to edit them? Oh yeah, you can create another solid to modify it, then union or subtract them again. Sometimes when the objects become too complex, the only thing you can do is start again from scratch!
Since AutoCAD 2007, Autodesk introduce ‘solid history’ to AutoCAD 3D modeling. It means that AutoCAD remember original forms before you did a boolean operation. Those object treated as subobjects.
See this illustration below. This 3D model was formed from a box and a cylinder. I still can change the cylinder diameter. How?
Just the same with how you select subobject, hold your [ctrl] then click object in 3d model that you want. After you select the subobject, you should see the grips. Move the grips. As simple as that!