Is it possible to change all node assigned on TermA to TermB?
The Views Bulk Operations module can achieve this in one fell swoop.

Build a page view that lists the titles of your nodes and is filtered to nodes of Term A. Add a field "bulk operations" (if on D7) and set it to modify entity values then select your term field that you want to bulk update. You'll then be able to mass update your nodes, and can replace Term A with Term B