This is not possible through the Moodle user interface.
If you are very careful, and don't screw up, then you may be able to do this sort of bulk update directly in the
database - but if you screw that up you could cause serious data-loss. (But a typical query might UPDATE mdl_qtype_multichoice SET answernumbering = 'none' WHERE answernumbering = 'abcd' AND categoryid = ???).
If you try that, you need to be aware that the changes won't show up until you go to Admin -> Plugins -> Caching -> Configuration, and Purge the 'Question definitions' cache.