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.