You've got the gist of it there Steve,
Basically the new terrain engine that the new maps (Nevada and Normandy) use cannot support the legacy Caucaus map in it's current iteration; hence the version 1.5 and 2.0 separation. Eventually the Caucasus is to be re-modelled within the new terrain engine and this where we will see the merge.
If you want to use Normandy you'll have to either
i) wait out the 2.5 merge (could be a good few months away yet)
or ii) download the 2.0 alpha and run it parallel to your current 1.5. You will then be able to update it to 2.1 when Normandy is released.
I suspect both installs will eventually reach 2.5 around the same time, and you'll end up with two identical installs; otherwise it could be that support for 1.x will cease and you'll need to start from a 2.x install.
I know - sproglet dependent! - that I shall be aquiring the 2.0 install soon and so have a parallel install ready to update as I'm gonna need to get some Normandy action in ASAP!
ASFAIK all modules available for 1.5.6 are operating in 2.0 currently...