The Interim Stage is an update to the initial release of Bliss Stage: ignition Stage. It is available only as a PDF. The next release will be the Acceleration Stage.
The changes are listed below per the designer:
There's no rules difference between the two texts. (ignition and interim)
There will be slight rules clarifications in the revised text, as well as a few extra options. The clarifications are:
1) On a relationship breaking, add double the Intimacy score to Bliss, not triple (the book right now sometimes says one, sometimes says the other).
2) Starting Bliss = pilot age.
By and large, I'm not expecting any serious rules changes. A lot of rewriting, but no serious rules changes.
The next release (Acceleration Stage) will include new Rules modules: One for online play, and one for tactical play, both of which are basically total rewrites of the rules.