You can only apply a work type to multiple pieces of work if:
Applying a work type to a work template
Once you've created a template, go to Details and click on Change work type.
When you select the work type, you can decide whether you want to apply it to all work already created off that template. If you don't check the box, only future work created from the template will be affected.
Applying a work type to a recurring work item (schedule)
If you are on a work item that is part of a schedule, if you change the work type you will be prompted to apply it to all work that's part of that schedule. f you don't check the box, only future work in the schedule will be affected.