Attaching a database (created in SQL Server 2012) to an instance of SQL Server 2014 without upgrading it I've a database in a production server (SQL Server 2012) and I've a development server (SQL Server 2014).

I need to attach the database in the development server but I've realized that once attached it's automatically upgraded to the version 2014 so I can't attach it again in the production server.

Is it possible to attach the database in the development server without upgrading it?

We are planning to make changes in dev, detach, and attach it back to production.
It is not possible for that upgrade to not happen. There are changes required for the database to be attached and usable on newer versions. 

This is under-the-hood stuff like internal references and pointer formats, the exact internal changes from version to version aren't documented. See here for some detail if only to satisfy your curiosity, but I would never risk trying to edit those bits directly.

It is possible to not change the compatibility level of the database, however you would still not be able to take a backup of that database and restore it to another SQL Server 2012 instance.

Because of differences in server/database versions your production and development servers should be on the same versions to really get the full benefit of testing in dev. If you aren't going to upgrade production soon, then you could downgrade dev.