Whenever a new ProShow version has been released, a common question is "Will this (Slide Style, Template, Transition) work in the newest version?" The answer will always be "Yes!", thanks to Backward Compatibility.
Any Style or Template that was created in an earlier version uses features that a later version already has. This is because the software builds on itself, so you can always be confident that an earlier Style or Template will always work in a later version. (Unless a feature becomes obsolete and is removed, which hasn't happened in ProShow yet....that's a rare occurrence).
Sometimes you can open a newer product in an older version, but you have to make sure that the item you are using doesn't utilize any features from the latest version. So, for example, if I were to create a Slide Style in ProShow 6 that doesn't use any Text Layers, it would still be compatible in ProShow 5.