How to change username?

Hello all

I have this big database and now I need to change ALL the users names.
Is there a way to rename a user name, or do I have to delete and add
it again (I DO NOT want to do this)?
I know how to change the password, but I have no idea about the user
name.

I do not want to delete then and add it again, because I will lost all
the data (I think)

Any ideas?

Thanks in advance
.

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