NEX reservation - ride earlier than reserved

I have a nex ticket with seat reserved (date and time). Is it possible to ride the NEX train that is earlier on my reaervation ticket? Say, I ride the 3pm train instead of the original schedule of my reservation which is 4pm?

NEX seats are all reserved seats, you need to change you reservations at the customer service counter, but NEX are hardly full.

You should change your reservation. On a paid ticket you should be able to change once. On a JRP you could either change or get a new ticket because the reservations are free (pre-paid).
If you get on the earlier train, the conductor will inquire/inspect your ticket and may re-issue it for you (and allocate a seat for you).
There are rules for riding the N'Ex without seat reservations, but normally they are more complex than the average traveler needs.

Is it possible to ride the NEX train that is earlier on my reservation ticket?

I once asked the NEX train conductor if it is possible to ride without reservation with JRP, the answer was surprisingly yes, and standing only if the seats were all taken. However, I have never experienced a full NEX in my many trips to Japan. And interestingly I have experienced our seats taken by others, and when we produced our tickets the persons just moved to the next available seats.

As our guest above says, that's exactly how it works in all-reserved-seat trains. Not just the N'EX, but every reservation-only JR train. So when someone shows a reservation ticket for the seat you've taken, you need to move. It's surprisingly un-strict with Japanese standards :)

Just to make sure: to have a Narita Express train ride, you usually need not only a reserved-seat ticket but also a basic-fare ticket or rail pass which is good for the section.

Not just the N'EX, but every reservation-only JR train.

That�fs a misconception.

Actually in JR trains for alll Shinkansen and most other Limited Expess services, any seat which can be reserved does always require reservation, i.e. you are not allowed to use it without reservation.

Narita Express is among only the four Limited Express services for which the peculiar system (mentioned by the unnamed respondent) works:
you can choose a ticket not specifying a seat, which allows you to take a vacant seat and use it, unless a passenger holding a valid reserved-seat ticket (for that seat) comes and appears.

Tickets of these two types are sold at the same price.
This system is unique to JR-EAST.

With the JPR on the N'Ex, yes - of course you can ride without an assigned seat. The rules changed in 2016 and are actually pretty clear, just not reflected in things like station announcements.
What I do see are people that board the train with the only the base fare ticket and then end up at Narita and have to pay the express fee - fumbling around for extra cash and holding things up for people with rail passes and valid tickets.
Also, I have had a few full (crammed full, 10-20 people standing in every car full) trips on the N'Ex. It does happen.