Alternatively, can I select all the objects with MNT-BY=<something> ??

whois -ri mnt-by <your-mnt-obj-name-here>

How can I change the MNT-BY attribute? Delete, and set up the new one?

I'd register the new one, move the objects under the control of the new one, double-check, and then delete the old one. Regards, Wilfried.