Transfer ERD between Databases on SQL Server
It sounds as if you are using the Database Diagrams feature from within SSMS. I found this tool out at CodeProject: http://www.codeproject.com/Articles/15080/Script-SQL-Server-2005-diagrams-to-a-file

I cannot speak to how well it would work for you. I would suggest you use a different tool, such as perhaps Visio, in order to create your ERDs so that they are more portable.