Like Adam already explained every update of EPESI core will overwrite your changes. You should not modify core modules, otherwise you will need to merge your modifications after every update. One method is to copy existing module into a separate module and modify it there creating your own version. This is legal and allowed under MIT license - you still need to provide proper credit in the info about the module, namely proper copyright notice. If you modify commercial Premium modules you need to check the license because paid modules are released often under other than MIT license.
But there is a better way to do it:
In modules directory you see subdirectory Custom. This is where your custom modules should go. If you intend to sell this module you can put it into Premium Directory. Create your own catalog where you supply all your stuff.
The database is altered during the upgrade process sometimes, but data is not changed. Therefore you make changes that are permanent on the database level. Record Browser provides for example mechanism to intercept original module's functionality by providing post-processing methods.
If you develop your module in Custom directory and then decide to move it to Premium directory you will need to refactor your code.