You can rename attributes etc. and the Mendix model will change your database accordingly. The only things you have to watch out for is changing the type. Changing an string to date time will offcourse loose your data. But my guess is that you would have very lousy developers if they are doing that. Same is true for deleting references or attributes etc. When you delete those in your model they will also be gone in the database. Adding attributes, entities and references are never a problem.
Hope this helps.
You may be interested in this learning path: https://gettingstarted.mendixcloud.com/link/path/106/Advanced-Domain-Model-Skills
It handles doing data conversions.