The default preference for saving Photoshop files is with "Maximize File Compatility" on. This means that it can be used with earlier versions of Photoshop. You also need to have it on for other applications which need to read it - such as Poser. So I can read all my Photoshop CC files with all the apps which support the PSD format.

If you stop paying, the software will stop working. You keep all your local files, but I assume that if you use cloud storage, you need to save them from there.