To save from the newer version to use in an earlier version, choose File > Save > Format: InDesign CS4 or later (IDML) to save an IDML file. Open the IDML file in the older version.
Installing a free trial of the new version should have no effect on your prior version.
Once a file is saved in the newer version, it cannot be opened in the older one. You can export it as IDML and open that, if your old version is CS4 or newer. If you do that, be aware that text may reflow, and any new features used will be lost or flattened.