Hello,
When changing the IP address (Manage IP) of the bgroup from x.x.x.a/24 to x.x.x.b/24 through webGUI, device will throw a message that they can not be same.
If you still go ahead with the configuration, for me I was able to see that it took effect and both manage-ip & interface ip of bgroup were set to 45.45.45.45/24 (for test).
Then I changed it to 45.45.45.46(both manage-ip & interface IP) & it took it without error.
Regards,
Rushi