To solve all your queries for today and for me to share accurate resolution and help you out in resolving all your concerns, may I ask if you can share more details about changing the second accounting firm of your client?
In the meantime, if you are referring to changing the role of the other accountant in your client's user's list, I'll be happy to help you how it's done.
This way, you'll be able to manage the other accountant's access and make changes if needed.
In addition, I've included a detailed guide for managing users access: How to Manage an Accountant User in QuickBooks Online?
Please let me know if you have any other questions or if you're referring to something else. I'll be waiting for your response and will help you from there.