In SharePoint 2007, upgrading a Web Part to a new version number was a bit complicated. But as Maurice Prather nicely explains in this article, this has become much simpler in SharePoint 2010. Although, due to a small bug, again described by Maurice in another great article, taking advantage of this in existing projects requires a bit of luck. If your public key token contains one or more byte pairs with a leading 0, you are not so lucky. As Maurice explains, the 0 will be stripped from the PublicKeyToken attribute of the BindingRedirect element in web.config. This will of course cause the binding redirect to fail, and the Web Part will render an error message.
I was a bit surprised to see that this was not fixed in SP1, but luckily we didn’t have to wait that much longer. I have just tested this after installing the June CU and now it works as it should! I followed these steps: