Is it possible to change the cardinality of an existing relationship class in enterprise geodatabase?
I want to alter a 1-m relationship class to m-m
No, unfortunately, it is impossible to modify the cardinality of an existing relationship class in any geodatabase.
From the Viewing and modifying relationship properties documentation:
You can't modify any of the properties you specified when you created the relationship class except for renaming it. You can set and change relationship rules at your discretion. If you need to change other properties, it is easy to delete the relationship class and re-create it.