Would adapting a traditional opera story into a novel involve copyright infringement?

No, myths and folklore do not have a specific signed author in the sense of copyright law, there is no copyright. Even if there is, the duration of the copyright, simply put, for individuals, is fifty years after death, the duration of the right of authorship and other moral rights unlimited; for units and legal persons, is fifty years after the first publication of the work.

For example, the Eight Immortals crossing the sea you mentioned is a Ming Dynasty legend, which has long since passed the term of copyright protection today.

Many opera plays themselves are derived from historical masterpieces, such as the Three Kingdoms play Water Margin play. Leaving aside the infringement of copyright, these stories have been y rooted in well-established novels. Re-adaptation is not much new. Unless the change of like the big talk westward journey of the spirit ......, completely with the fixed impression of people's minds decoupled.