It is a fairly simple process to generate a plan above your already modeled 2D first floor plan. What you have described is probably the way most users approach developing a multistory building.
I suggest grouping the first floor plan and then using the Move tool to copy the plan along the blue axis (straight up) for a distance of 10 feet. You could then edit the upper floor plan independent of the original lower floor. If you find it distracting to work on the plan while it hovers above your original plan, cut the upper floor plan, paste in place to a new file. Perform your modeling on that file, then cut it and paste in place back to the original file location.