alzZed,
Sounds like the gear selector plate may require adjustment.
You will find it at the base of the gear lever and you will get access from inside the car.
This will have been disturbed when they were swopping out the fly.
Alex.
As previous quoted, a transmission fluid change is best tried first.
Fresh genuine Nissan fluids and moly have had considerable success on this forum.
Genuine Nissan fluids and DC Moly in stock and good to go same/next day.
Alex.
There will probably still be some air trapped in the system.
With the lid off and engine running, turn the steering wheel lock to lock about a dozen times, keeping the fluid topped up.
Alex.