Select an existing tag to replace with another one;
two or more to consolidate into one. The required new tag
may actually be an existing tag or a new one. A change comment will
be added if supported by the tag storage for the resource's realm.
Beware: Attempting to replace one or more tags will
always add the new tag, but removal could fail
for some tags, because tags might be immutable depending
on internals of the tag provider they've been derived from.
You'll need to investigate further on how to change/remove these
remaining tags closer to the tagged resource, i.e. in your
trac.ini see ticket_fields option
that lists fields to expose as tags for tag realm:ticket.