You want to pull your DB into a SQL Project. Maintain all of your changes there. This tells your system what the schema of your database should be. From there, I'd generate the dacpac files (through building the project) and provide those to your clients along with having them install the SSDT tools that include SQLPackage. They can run SQLPackage to make changes to their database to handle the schema changes automatically. This will bring their database in line with your schema, no matter how far off it might be.
I'd also create a publish profile for them to use. This lets you control some of the settings.
- You can choose to not drop any objects not in your project
- You can choose to ignore users/permissions
- You can set an option to not allow changes if there would be data loss.
- You can wrap everything in a transaction so a failed update rolls back
- If you give them a batch file to run, you can specify an output file or a Diff report, or have them generate their own script to do the update.
I blogged about this at http://schottsql.blogspot.com/2013/10/all-ssdt-articles.html (or http://schottsql.blogspot.com/search/label/SSDT if that doesn't work well). That will take you through some basics of why you might want to use SQL Projects, creating them, maintaining them, and publishing the changes to an existing database.