NOW sometimes, the conductor might let someone do it, other times no.

There is no penalty or anything, but it means that someone else may have to stand despite there being a vacant seat, so you should make the effort to catch the train you are reserved on or else return the unused seat reservation to the ticket counter so that the seat can be released for someone else further down the line.

Then, your seat will remain vacant, which could cause some minor inconvenience to other travelers and the railway company. As for yourself, you will have to make a reservation for another train or ride in the unreserved section of any other train.

It is possible to cancel reserved seats before departure: we arrived earlier than expected for a transfer and were able to cancel the seats we had and obtain seats in the earlier train at a moments notice, with about 5 minutes to spare: don't recommend cutting it that fine but it worked for us at the time (heading home from Narita: managed earlier NEx transfer at Shinagawa)