Is there a term for coining a phrase for a word that already exists?

Usually a new term emerges and it becomes necessary to add a qualification to an old word for disambiguation.

examples:

• "mono sound" versus "stereo sound"
• "analog watch" versus "digital watch"
• "acoustic guitar" versus "electric guitar"
• "slow food" versus "fast food"

Solution 1:

You have named textbook cases of the retronym, which AHD defines as

A word or phrase created because an existing term that was once used alone needs to be distinguished from a term referring to a new development, as acoustic guitar in contrast to electric guitar or analog watch in contrast to digital watch.

Language Log and others ascribe the term ca. 1980 to Frank Mankiewicz, a writer and Democratic political strategist who was later head of National Public Radio. You can find a large collection of them at Retronyms.org.

For a related phenomenon, see Word that means “outdated name”.