Yes, it should be compatible.
If you accidentally include V9 new features the model will still work in V8 but absent the new features. How much of a problem that might give rather depends on which features they were and how important! It is probable that a failing V9 model used in V8 could in many circumstances be edited to adopt V8 methods of doing the things that have dedicated functionality in V9. Not a completely good result but maybe better than re-creating and entire model.