Can I move an attic furnace downstairs
No, not really. The work you're describing doesn't sound remotely cost-effective, or effective at all for that matter. Running a huge duct from the basement to the attic will choke off the airflow. Not to mention that if you're going to to that, you're already doing half the work necessary to redo the ductwork properly to connect the two zones.

It sounds like the real solution would be to abandon the attic unit (possibly selling it for some quick cash if it still works), and then integrate the second floor ductwork with the first floor ductwork such that the basement-mounted unit can service the whole house adequately.

If your second floor gets really really hot, the first and probably most consequential step is to air-seal and super-insulate the attic floor, and possibly adding a radiant barrier under the roof rafters if you can perform the work yourself (don't hire that part out; the payback time will be on the order of a hundred years). Low-E storm windows can help too if your house's windows are old and crappy.