So ...

Upon activation, go to Tools => Bulk Move.

All you need to do is select the targeted category (A) in the left drop-down menu and the new category (B) in the right drop-down menu:

And that's all ... When you click the "Bulk Move" button, the plugin will transfer all your posts from category A to category B. Simple as that ...

Similarly, you can also move posts between tags ... Simply choose a tag, whose posts you want to move and then select the tag where you want posts to be moved.

Cool, isn't it?

OK. Now let's see some closing thoughts ...