Hi tinie,
Actually I think you mean you installed the upgraded version of the SS06 pack that you downloaded through the forum, right? The free upgrade for SS06 addon customers?
In that case, or in any case, you can just install NH over it and it wont clash.
But, if you allready have the 6 ships, then you can also just buy the 2 new ships and cornwall, when they come out as seperate addons in April. You don't really need to buy the whole NH pack if you don't want to, because you allready have a part of it now. (You can of course, if you dont want to wait)
The 6 ships you allready have will be updated with a free patch that contains an update for the ships and the missions. And when you install that then the 6 ships will be exactly the same as the 6 similar ones in the NH addon.
And the content that was not in the SS06 addon yet, so the bugsier 2, the pushboat and Cornwall, will be available to you then in single downloadable addons. (and those wont be free then of course) But that will take a bit more time of course. So unless you don't want to wait, you can decide to buy JUST the stuff you don't allready have/get a patch for.
But in any case, there will be no issue with installing NH over the stuff you now have installed.
Regards,
Fred