Older models can be opened in Poseidon 3.1 and later, but they will be saved in the Poseidon 3.1 format so they cannot subsequently be opened again in a version below 3.1. Before Poseidon opens the model, you will see a warning box.
Upon saving, Poseidon will automatically make a backup copy of your original model. This backup can be opened in pre-3.1 versions, but will not contain revisions made with 3.1. For extra peace of mind, you can always manually copy your original project.