Format issue

There's no possibility, and no need to save it in a special way. The Express edition will open all files of the same version as express, and all previous versions. Prime 12 express will open all Prime 12, 11, 10….1 files, but not any prime 13 (or higher) files, once they exist. And it will show the results of all calculations as last saved.

Any limitations are in the functions that the express edition will execute/work with:

The Express edition will not (re-)calculate any program structures, symbolics, plots outside of the simple X-Y plot, and solve blocks. And there’s a (quite) large bunch of functions that it will not  (re-) calculate, among which are: Re(), Im(), mean(), stdev() and linterp(). But there is a way around many of those, see here.

Success!
Luc