AutoCAD Inventor :: Fundamental Change Of Geometry Near The End Of Project?

I am designing a vacuum part in Autodesk Inventor, which is essentially a long cuboid shape with various valves and spaces for windows along it's length (see attached pic). Upon finishing the design of this part, a few details need to be changed - one being the fundamental shape of the object. It now needs to be a larger square section attached to a long cuboid section (as I found out the valves on the right hand side of the drawing are around double the size they are in this drawing). Rather than restart the project from scratch, I would like to only modify one of the ends to be extruded out into a square shape, or something like that. My first idea was to use the split command in order to split the faces into what I did and didn't want to change, but upon starting this, I don't think this is the best way.