I want to be able to make changes to the method signatures, names, etc on existing SignalR hubs. Is there a defined strategy for versioning SignalR hubs, so that old JS code can continue to work, without having to create a fresh newly named hub for the new calls?
I'd say the short answer is no. Signalr does not have any versioning strategy like that.
The question is a little light on details, but signalr has for instance the option to use an attribute on a class to define the hubname. You could also create a new hub class by deriving from the original Hub in order to help you reuse existing code. Whatever you do new clientside js will need to be written.
You can't change the method name and expect Javascript to understand that you changed its name. As for method signatures, Javascript is not a strongly typed programming language, so you can pass different types of variables to the hub (as long as the number of variables stays the same).
However, you can create new methods, if you want to continue to support the old ones.