Is it possible to change the default value or a field in an eti without messing up the production database? There is a drop down that defaults to other that I would like to default to another value in the dropdown. I don't want to mess up the database though. Is there another way to set the default value on the drop down besides the eti file it's self?
Why would you mess the DB?
Default value in SQL actually sets this value for the fields that are NULL during input. This shouldn't require a DB drop and shouldn't break anything.
As for changing this. If that's not OOTB entity, you should be able to do that in .eti.
As to your use case - what are the values in the drop down? I would assume that's a typelist, and that's what you really should modify (look for priorities there).
EDIT
To override a column for example for OOTB entity User.eti
Open User.etx or create it if it doesn't exists. modules\configuration\config\extensions\entity\User.etx.
Right click on a column that you would like to override - ExternalUser in your case. Pick override.
at the top of the list a column-override should be created - change the default value there.