No it is not possible to convert back to an older version, we can't write code in the modeler to be able to convert a model that we don't even know is possible to create yet because it will be made in the future. I do wonder why your customer doesn't simply upgrade their environment to 4.8.3 though, that would be the most obvious solution. Otherwise you'll just have to re-apply all your changes you made to the old 4.7.1 model.
The following solution is not recommended; but you can import an old file! The .mpr file is a SQL light database. You can open the .mpr file with https://github.com/sqlitebrowser/sqlitebrowser
There, you will find the version. Change your version number and save and you are able to open the file.
Be really careful; Mendix is not Backwards compatible and you might break your system. On minor version difference you most likely to make it work... but I is at your own risk.