I thought I'd take a crack at making tag wikis for them since I have an intuition of what they mean but for some help with a precise technical definition of each I looked up Wikipedia and found this for Historical linguistics:
Historical linguistics (also called diachronic linguistics) is the study of language change. It has five main concerns:
That seems to make it clear we should merge them but then a few sentences into the page for Language change we have this:
Two linguistic disciplines in particular concern themselves with studying language change: historical linguistics and sociolinguistics.
Whatever the answer to that, what should we do about these two (or three) overlapping tags?