In article ,
says...

Is it not replacing an existing module though?

Yes, the Russian Multipurpose Laboratory Module (MLM) will replace the
Pirs Docking Compartment. I believe that the MLM was based on hardware
which was originally to become FGB-2. From what I remember, the US
insisted on a backup in case there was a problem with FGB-1. Once FGB-1
was successfully launched, work on the module, configured as FGB-2, was
suspended.

Hmm, could this be the space version of the everlasting Broom, its only had
ten handles and three heads.

Not really. There are key "core" ISS modules which would be hard, to
extremely difficult, to swap. In addition to a link to a picture (at
the bottom of this post), I'll attempt to describe the configuration
using words.

On the US side, for example, the US truss is attached to the Destiny
laboratory. I'd imagine replacing Destiny would be quite difficult,
partly due to the truss, and partly because it's attached to both Node `
1 and Node 2, which are each attached to other modules.

On the Russian side, the Zarya Control Module (FGB-1) and the Zvezda
Service Module (DOS-8, a Salyut style module originally intended to be
the core module for Mir-2) are not only connected to each other, but are
also connected to the US side and to several other Russian modules.

So, the "core" of ISS consists of the following modules, essentially all
cylinders whose axes are all aligned along a central axis, in the
following order: Zvezda, Zarya, PMA-1, Node 1, US Lab, Node 2, and PMA-
2. In addition to the truss being attached to the US Lab module, all
other modules are attached radially to: Zvezda, Zarya, Node 1, and Node
2.

A pictu
http://upload.wikimedia.org/wikipedi...ain_ISS_config
..jpg

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer