Theory

Opening

The project file records the version and the file-format revision that wrote it. On opening, this version converts the settings and the file layout step by step to its own format, in memory. A short message says that the project came from another version.

What a conversion can do

*Converted*: a setting was moved or renamed in a later version. Nothing is lost; the dialog lists these.
*Not kept*: a setting or a result type that was removed in a later version. This version cannot use it, and a save leaves it out; the dialog lists each one with the version that removed it.

Saving

The first save of a project from another version asks first, and lists what is converted and what is not kept:

*Save as a new file* writes the converted project to a new file and leaves the original untouched.
*Save and convert* overwrites the original; the original is first copied next to it as <name>.<old version>.bak.spsim, so it can still be opened with the old version.

A converted project may not open in older versions.

Files from a newer version

A file saved by a newer version opens when that version only added things (new settings, new result types); this version leaves the additions out and says so before saving. A newer version that changed the file layout itself marks its files so that older versions refuse them, with a message naming the version needed.