Air Conditioning - zoning AC unit in a townhome

I have a three story townhome in Houston TX- 2500 Sq feet. (apprx 800 sq each floor)
First floor has its own AC unit. Cools fine- no problem.
Second and third floor has a single unit. Second floor has a 24x36 air return with 8 registers. Third floor has a 12 x 24 air return and 3 registers.
Third floor is the smallest of the floors. The house cools real nice and is always very comfortable. I want to make the system more efficient by retrofitting some zoning. We use third floor only for guests. No point in always cooling it down.
My question regarding two ideas that have come to me.
1- Can I just block of the return and registers on third floor? Will this option stress the blower motor?

2- Install a second air return on the second floor (feeding into the air return from third floor). Install two more registers on second floor (coming of the third floor). flip a switch and a controller will either divert air to/from third floor or second floor.

possible?

Id say let it alone . Sounds like they did a good job when they put it in. You say it works just fine. The only way is to close off the stairs put insulation in the floor . So the hot air from down stairs cant get up to the top floor . Then the unit will be to big and not run enough to get the humidity out for you. So I say let it alone. You wont save that much. ;)

with Ed on this one!!