Well, there's a few ways.
(1) Synonymising without merging: questions tagged [historical-accuracy] stay that way but any future questions with that tag will be mapped to [historical-context].
(2) Merge-synonymisation: all those questions are tagged just [historical-context] and anyone typing "historical accuracy" in the tag box in future will get [historical-context].
(3) Merging without synonymising: all existing questions are tagged just [historical-context] but people may not find that tag in future by typing "historical accuracy".