I used the feature "Change Several Bugs at Once":
- pressed "Check All"
- near the bottom I selected "Reassign bugs to skasal"
- and pressed "Save Changes"

Bugzilla answered with a red message:
 Um, you apparently did not change anything on the selected bugs.

I had to work around this: add a short comment.

(If it is intentions to force user to add a comment, then the message is not
very descriptive. But I do not think a comment should be required when
maintainer changes.)