Version 1.0 will be able to open any pxo file saved starting version 0.8 and on, so you can still open your old projects. However, pxo files saved with version 1.0 cannot be opened by any earlier version, because the format has changed.
Also, version 1.0 will not be able to open palettes, shortcut profiles and extensions made for 0.x, and vice versa.