Apologies if this has been answered elsewhere but if I open an X3e project in Platinum and make changes and then save it can I reopen it in X3e.  Is there any backward compatibility ?

Typically there is risk in going backwards, especially if you use some of the new features in Platinum.  Just out of curiosity why would you want to go back after making changes in Platinum?

dwardzala
Typically there is risk in going backwards, especially if you use some of the new features in Platinum.  Just out of curiosity why would you want to go back after making changes in Platinum?

Maybe he wants to purchase monthly and is worried if he decides he doesn't like it?
 
@OP, yes with dwardzala's caveat.
 

Last night I successfully open an x3 project in x3, that had been saved in platinum. No problems, no warning. I was surprised.

There has always been that ability to my knowledge (as severely limited as it is). The only thing you should have to worry about is if there are any Platinum "only" plugins/features involved, in which case you could get a warning stating such.

Platinum projects will be fully compatible with X3 as long as you don't use some of the newer features like VocalSync and MixRecall. Also Craig's new prochannel amps won't work there.
The projects will still open fine in X3 but they will be missing the new features from Platinum.

 
It works fine, the only time you'll get a warning is if the project you're trying to open in X3 contains anything that is Platinum-specific.
 
For example, I love the new Pro-Channel ReMatrix Solo convolution reverb Module, so I have edited all my custom Templates to have it inserted on all the relevant busses.
 
When I open a project in X3 that was created in Platinum and has ReMatrix in it, I receive this message (obviously they need to update the warning to show "Platinum" instead of "X4"):
 

It appears Overloud did not get the memo.