The "Tools" drop-down contains the option to "Auto-Calculate WBS" if the WBS column is displayed and you are a project manager on the project. This option reassigns all current project tasks with the 1.0, 1.1, 1.1.1, etc. WBS format. It is intended for the use of setting all current task WBS assignments, while any new task would be need to be assigned a WBS ID manually.
If you apply this utility to a task list which already has numbers, ALL tasks get renumbered, which could mean that existing tasks get a WBS ID change. It warns you with a dialogue box before it proceeds.