You can export the 2.5.6 module, fake its version number and import it into 2.5.3. Faking a version number can be done using the add-on SQLite Manager in Firefox. You can open the .mpr file with it (be sure to select 'All files (.)' to actually see it) and manipulate its contents. The table you need to update is _Metadata and you can set the _ProductVersion column of the single row to 2.5.0.
Note that no guarantees can be given when 'hacking' an mpr like this but it might save you some work.
No. But if its about an appstore module, older versions might be available as well.
Thanks, Michel. I need to use it both for integrating an appstore module (questionnaire, oldest Mendix version 126.96.36.199) and to integrate functionality developed in 2.5.6.