This article describes how to add a Zoom Meeting breakout room after it has already been assigned if you are unable to do so. It includes a guide for updating Zoom which is a prerequisite for adding the rooms after they have already been created and assigned.
1. Open your Zoom client. Click on your photo in the upper right. Select “Check for Updates”.
2. If you have an update available it will download and then ask to proceed. You want to be on version 5.3.1.
3. Once on this version you will be able to add breakout rooms on the fly – even after you have already created & assigned rooms!