X2 will load all X1 projects just fine and generally saved X2 files will also open in X1 minus the new features.
For example if you save an X2 file with duplicate PC modules X1 will load only one instance. As other said, unless you plan on going back and forth between X1 and X2 for some reason such as sharing with a non X1 user, its generally good to save the X2 version under a new name.
If you do plan on going back and forth avoid using X2 specific features and you should be fine.