Hi,

There is a website v1 on http://www.mydomaine.com.
For simple WordPress, I give a directory like mydomaine.com/v2, do the stuff, setup plugins, upload/write content and when it’s ready (few weeks after), I change the index and rewrite the permalink like here : https://codex.wordpress.org/Giving_WordPress_Its_Own_Directory

For MU, I need to
1/ Install as simple WP in http://www.mydomaine.com/mu
2/ do the Giving_WordPress_Its_Own_Directory
3/ Create the NetWork
But there is a webiste on http://www.mydomaine.com.

Is it easy to do
1/ Install a simple WP in http://www.mydomaine.com/mu (v2 beta)
2/ Create the NetWork
3/ Work on it
4/ Erase/Back Up the v1 website in www
5/ Giving_WordPress_Its_Own_Directory and Pointing http://www.mydomaine.com to /mu