How can I re-bootstrap pkgsrc?

[Roland Illig <rillig%NetBSD.org@localhost>]

Hi,

I am using pkgsrc as an unprivileged user, and I want to update pkg_* 
and bmake to the current versions. So I ran bootstrap/bootstrap, but it 
failed:

$ bootstrap/bootstrap --unprivileged --prefix=/usr/pkg/2007Q2
...
===> Installing for bootstrap-mk-files-20071104
ERROR: bootstrap-mk-files-20070810 is already installed - perhaps an 
older version
...

How can I update the pkgsrc "base" packages?

Roland