Hello EtienneHB
Thank you for posting in Microsoft Community forum.
You should redirect user folders back to their local machine and then redirect to new server.
Here is a thread with steps that you can redirect user folders back to their local machine.
how to remove Server Folders and demote DC - Microsoft Q&A
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou