The EDMX file is an XML file that is a combination of 3 different parts that make up the whole thing. If you right-click on your EDMX file and choose "Open with... XML Editor" you'll see the 3 different sections:
<edmx:ConceptualModels>
<edmx:StorageModels>
<edmx:Mappings>
These sections can be edited manually, at your own risk! :-)
That way you can modify only what you need to.
Note that it's also possible to generate CSDL, SSDL & MSL files rather than having them embedded in the binary file, by changing the "Meta Artifact Processing" property of your model to "Copy to Output Directory".
If you don't want to do this manually, there's the Huagati DBML/EDMX tool, it is free and you can download it from huagati official site or from visual studio gallery, which is a Visual Studio plugin that allows you to select what changes need to be done.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…