[SalesForce] Re-set user.IsPortalEnabled field

I can't figure out how to set IsPortalEnabled to true, once it has been set to false. Not a critical issue, just trying to increase my knowledge and understanding of the platform.

I was playing around on my dev org and ran:

list<user> lst = new list<user>();
for(user u :[SELECT id, name , IsPortalEnabled, isActive FROM User where IsPortalEnabled=true]){
   u.Isportalenabled=false; 
   lst.add(u);
}
update lst;

which disabled my portal user and set him to active, trying to reactivate the user in through the gui I got "Error: Cannot activate a disabled portal user" which is understandable.

Trying to reverse my actions, I ran:

list<user> lst = new list<user>();

for(user u :[SELECT id, name , IsPortalEnabled, isActive FROM User where id = '005d0000001DXMx']){
    u.Isportalenabled=true;
    lst.add(u);
}
update lst;

which gave an dml exception

EXCEPTION: System.DmlException: Update failed. First exception on row 0 with id 005d0000001DXMxAAO; first error: INVALID_CROSS_REFERENCE_KEY, Cannot enable a disabled portal user: [IsPortalEnabled]
STACKTRACE: AnonymousBlock: line 7, column 1

Trying to re-enable the portal user through the button on the contact detail page, just ended up in creating a new user.

Anyone who can shed some light on this, is it totally not possible to reverse setting IsPortalEnabled to false ?