portsnap update won't update original /usr/ports

[Gary Aitken]

According to the handbook, one can do
   portsnap fetch
   portsnap update
and the update will work with a previously created ports tree;
I presume this includes one created during system install.

However, when I attempted this, portsnap complained:
   /usr/ports was not created by portsnap.
   You must run 'portsnap extract' before running 'portsnap update'.

Is there a way to use portsnap against this tree, or must I delete the 
existing /usr/ports and do an extract first?

Thanks,

Gary