What is musician work for hire?

A musician work for hire is when a musician is hired to perform music, such as on a recording, but agrees that he or she will own no copyright or moral rights on the music. In the U.S., a work for hire contract must be in writing, must state clearly and in big print that it is a work for hire contract, and the contract must be offered and signed before the work is done. I do not know how this relates to the second part of your question. If you recorded an album with a band, the contracts you had then control what you can do with the music. If there were no formal written contracts, maybe there was a contract that was formed by emails or letters. The contract with the musicians and others who worked on the album will control what you can do or what you will need to do to proceed ahead. You may need to make new contracts with them to cover this new remastering. You need to show all the contracts to a music lawyer and let them help you.