I'm not sure that I see the purpose of the following two lines anyway since the result will be to asign the Username value back to itself:

Dim usern As String = u.Username
u.Membership.Username = usern

In DNN 5.1 and later, it is the UserInfo.Membership.Username which was deprecated, not UserInfo.Username. In the code for the deprecated Username property (UserMembership.vb) we see:

<Obsolete("Deprecated in DNN 5.1")> _
<Browsable(False)> Public Property Username() As String
Get
   Return _User.Username
End Get
Set(ByVal Value As String)
_User.Username = Value
End Set
End Property

Which simply gets and sets the UserInfo.Username property.

One other point to watch here is that GetCurrentUserInfo will allways return a non null UserInfo object even for the unauthenticated user. Rather than checking for a null UserInfo object I would check for u.UserId <> -1.

 
Yep. Remember this is pre-DNN 5 leftover. So the question would be wether it made sense under DNN 3 and 4. A lot of these things are probably work-arounds for bugs in earlier code.

Another explenation is that this is a remainder of a debug session: check if this is the correct user. But then, why write it back?

I'll find the answer, trust me.