Not a good chance. If you upgrade and then want to downgrade, the flight will have to have the same coach inventory available. I believe there may also be change fees applicable.
For only 4 J seats I think you are playing with fire thinking of doing an EXP upgrade. I'd use the eVIP as on that flight, depending on when you bought your ticket the chance of being in the "top 4" is probably petty slim. YMMV