I've been happy with Photoshop CS2 for a long while now, but I've recently upgraded my camera, and I've found that the RAW plugin I've got doesn't support the updated version of the RAW files. Is it possible to upgrade to a later RAW plugin, without having to fork out for a later edition of Photoshop?
Yes and no. The interface between ACR and Photoshop has changed a couple of times over the years. When a change happens, Ps can only use versions of ACR up to when the change took place.
That limits the possibilities for updates, but some updates are still possible. Adobe has a download page with updates for the various versions of Ps.
That said, chances are that it won't be sufficient. The most recent update to ACR for CS2 is still pretty old, so even though you can probably update somewhat from what you're using now, it still won't be enough newer to let you load files from a brand new camera.
Unfortunately not. Adobe in their wisdom decided to force you to upgrade Photoshop if you splash out on a new camera model.
You may want to download the free Adobe RAW converter, and convert the images to DNG format, which will be compatible with Photoshop.