How to change for (existing) System User for a domain/web space?

Hello all,

First of all thank you for reading my post.
I wanted to ask you, as the title says, how to change the current system user of a specific domain/web space for another existing system user?.
I currently go to "Web Hosting Access" and from there I try to set the new (existing) different user but it says it already exists.

Any help?

Thank you in advance,kind regards.