Set-ADUser –Identity $usr -add @{"extensionattribute1"="test"} # this work

Set-ADUser –Identity $usr -add @{"extensionattribute1"="test2"} # this not!

Set-ADUser –Identity $usr -replace @{"extensionattribute1"="test2"}

Set-ADUser –Identity $usr -clear "extensionattribute1"
Set-ADUser –Identity $usr -add @{"extensionattribute1"="test2"}