the only permanent thing that might need to be done is the enlargement of the hole for the piping to pass through of the driver side front fender. i haven't owned the NISMO CAI but typically with the AEM and INJEN that is the only permanent change. outside of that is usually 1 other screw used to attach the intake to the body..the rest are bands.
you could basically remove the OEM, put in the NISMO, and vice versa in minutes if you know what you're doing. the only clue left behind of there being an aftermarket intake would be the size of the hole in the front fender if someone knew what they were looking for. but with the OEM in place, the hole is pretty much blocked from view.
If you get the Nismo Gen-II CAI specially made for 2004 only, it is a bit harder AND careful to install because you have to dismount the MAF assembly and put only the precious sensor into the intake piping hole already there for this.