I think only the Adobe guys can answer this.
i hate to say it, but not even those guys can answer it
Just to make sure we're talking in the same terms, backward compatibility means that a newer version is able to open projects created in earlier versions; forward compatibility involves older versions being able to work with projects saved in a new version.
The impending Premiere Pro update is backwards compatible with prior versions. I just opened projects last saved in the current CC version, CS6.0.3, and CS5.0.3 in the current development build. That does not necessarily mean that every detail of a project will come through exactly as it existed in its original form--the magnitude of testing that would be required to make such a guarantee is staggering.
As for forward compatibility, CC projects will not open in CS6 or earlier; I can't really speak definitively to whether projects from the impending release will work in the current release. While I can testify that a project saved by the current development build successfully opened in 7.0.1, that project does not exercise the whole slate of new features.
Point made well Mark
I will rephrase a little and offer an example regarding planning.
I start shooting (next week) and will be editing a project that will take about one month to complete.
During this time an update is planned to CC.
I can do the update mid project or not.
Now some updates in the past are exactly that...updates ...and they dont get into version issues of forwards , backwards or sidewards. Some were called upgrades and usually paid as such.
The Cloud model is called updates and I doubt we will see the term upgrade again.
So..at what point does forwards , backwards or sidewards compatabilty come into it?
Maybe it doesnt even matter... unless one wishes to revist a project and maybe ..Importing projects will be enough for the task.
Maybe this is a thing of the past:
Previously I kept versions eg 5, 6, CC and I have always kept the Projects in Version specific Folders eg 'Premiere CS6 Projects', 'Premiere CC7 Projects'...
I know some one is going to say..dont update mid project.
Easier said than done when edit projects and productions frequently overlap.
You have got to pull the trigger sometime!
hmmm.. let me know what you end up deciding and how you safeguard yourself.. its an interesting question...be nice for others maybe as well, to see what they can do to backup projects in such a way that going back to them with newer versions, updates, upgrades ( whatever new stuff is called )...like specify how you import , do you use xml , backup with lossless at full res, how you save the original footage etc ??