Not really.. though I can see why you might initially think that....
The point is.. you can import LP9 projects into LPX so you can use the new features of LPX with your LP9 projects... but the import is one way for obvious reasons as LPX's new features are not available in LP9.. such as Drummer... so an LPX project cannot work using LP9 simply because LP9 doesn't have these new features in it.
There is one way you can use LPX projects in LP9 and that is to export Audio Stems out from LPX and import them into LP9 but of course then everything is exported as Audio only (inc SI tracks) and everything is fixed... (Post plugin) so that may not be practical in many cases.. Its the same method used when moving projects from one DAW to another...