You can change the content in the master version and you can re-name the master version to something else, but you can't swap the master to another version. 

For example:
You have 3 finished email versions: London, Berlin and Paris. London is your master but you want Berlin to be the master instead. If the content is already done, you cannot swap them over. You would have to rename your master version to Berlin and rebuild it with the Berlin content. Then do the same for London.  

Useful Links:
What is a master version? | How Versions work together | Non Master Versions | What is inheritance? 

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