Your older projects will still open, and will load the Legacy version of the plugin that you originally used in the project.
NOTE FOR FINAL CUT PRO X USERS:
Some Universe plugins may fail to apply when loading older FCP X projects. If you will be using a project that has plugins from 1.6 or before that you need to load, you will want to download and install this file to get the original versions of these plugins again. This will mean you will see older Universe plugins alongside newer 2.0 versions, which now all have the uni. prefix in front of their name. Going forward, we advise using the newer 2.0 versions.