Yes you can change them back with no problems.
A set of unrevised Z springs will run you ~$80-100. A set of good aftermarket ones will be at least twice that. Install will run you ~$100-200.

 

I would say go with the 350z springs. Do it yourself if you KNOW HOW. Its pretty simple IF you have the right tools.

Springs $100
Install $100 (don't pay more than that, it really should not take more than an hour or two if you have it done professionally).
Alignment...I don't know what this cost, my mechanic does it for free.

Removal of 350z and replacement with OEM springs, another $100.

So figure about $300-400 to lower it while you lease.

If you were to keep the car, I would recommend getting shocks as well since they help. But if you are getting rid of it at the end of the lease don't worry about it.