Hashbangs are only a stop-gap at this point - modern browsers ship with the HTML5 History API.

Does IE8 ship with the History API?

No, IE8 is not a modern browser by most definitions.

Which definitions? I've seen definitions of "modern browser" that only exclude IE 6 and 7.

They are wrong. They use modern to mean "quite new and available to most clients" instead of "actually implementing the majority of the latest standards".

IE 9 comes close, and can be considered modern.

I'm not sure what you mean by little snippets of javascript, a link is a link. If you change why couldn't you just 301 them to your new format? It's a trivial redirect that you're keeping for the g-juice.

The part after an hashbang is not sent to the server, so you need client side code to load the content.

This is because the # in an URL is supposed to be an identifier of a element in the page, not an actual resource.

Or you could just keep a tiny snippet of Javascript that searches for old style hashbangs and redirects all of them to the "another solution" handler to decide what to do with them. It can be just 3-4 lines of JS.

The problem is that you have to keep that snippet forever if you ever use the hashbangs. That's why I'm saying it's not a future-proof solution. Not that few bytes matters, though.