New software is often carefully calculated to reduce the value to consumers of the previous version. This is achieved by making programs upwardly compatible only; in other words, the new versions can read all the files of the old versions, but not the other way round. Someone holding the old version can communicate only with others using the old version. It is as if every generation of children came into the world speaking a completely different language from their parents. While they could understand their parents’ language, their parents could not understand theirs.
As an aside, this is one of the main reasons why writing in plain text is great. It’s future-proof. Not to mention simple, lightweight and with Markdown/MultiMarkdown, actually quite powerful (although no longer as simple…).