Taxonomy Replace
This module provides functionality to replace one taxonomy term with another. When a term is replaced, all references to the original term are updated to point to the new term, and the original term is then deleted.
This module allows taxonomy terms to be replaced by other terms.
Drupal 8 version
Taxonomy terms have a "Replace" tab, which allows users to select a taxonomy term in the same vocabulary.
Any references to the current term will be replaced with a reference to the selected term, and the current term will be deleted.
To replace terms, it is necessary to have the 'replace taxonomy terms' permission, and permission to delete the specified term.
Drupal 7 version
In the term edit page you can define another term or terms which will attached to node when node will be saved. Original term will be deleted.
Installation
Enable module and create term reference field with machine name which begin on field_taxonomy_replace in vocabulary where you want to use this feature.
The Drupal 7 version is not under active development.