Can I open a version of my app built with either 2.6 or 1.7 in Unity 3 and then reopen it again in the earlier version?
WARNING!! NO! DO NOT DO IT!
Once opened with Unity3 scenes are effectively destroyed for use in earlier versions, specifically in my case Unity2.6 and 1.7. Only copies that have not been opened with Unity3 can be opened by earlier versions.
It is not possible.
When you open a project from an older version of Unity with Unity 3, it says
"Upgrading project!
Your project was created with an older version of Unity.
Upgrading the project might take a few minutes."
[Continue] [Quit]
This completely changes the project's organization (in /Library/) to suit Unity 3, so the project will not work with older versions of Unity anymore.
But on the other hand, why use Unity 2.6 or 1.7 when you can use Unity 3? ;)
Well,I find many shaders used in earlier version don't work in 3.0,Such as AlphaDiffuse2sided and glass shader.