How to bulk re-assign licenses

All of our active users are assigned premium business licenses. We are trying to upgrade their licenses to E5 Security through a dynamic group. The problem is - it won't allow us to stack licenses. If we un-assign their premium business licenses, it will remove them from the dynamic group we created. We can't bulk assign after removing licenses, as many former employees are still in the system. Is there a way to bulk re-assign or upgrade the active users?