I did this:
3004.388.8 is running, first two mesh RT-AX88U_Pros to 3006_102.4, then the mesh master to 3006_102.4, then two mesh RT-AX88U_Pros to 3006_102.5, then the RT-AX88U_Pro to 3006_102.5, and everything with a dirty flash.
No problems encountered yet.
Thanks, RMerlin, for the good work...