You can't really re-use the hull but throw the speed requirement out the door as the hull is optimised for that range of performance - I agree that dropping the speed requirement would be sensible in hindsight but it's now a part of the design.
I suspect what will happen is that the ships will begin acquiring permanently fitted gear and become more like, the way the Stryker is used in the army - a base core of a ship, with some fitted out for a general role and others specialising.
One other thing that probably would cost a bit in the short run but would save on through costs would be to standardise the fit for both LCS in terms of CMS and so forth.
I believe LCS 2 has a soft patch for additional kit and that adding Mk41 wouldn't be difficult.
http://uglyships.files.wordpress.com/2009/07/lcs-gd-line2.gif
You can see the patch just aft of the gun. Stick an illuminator or two in there or better yet some of the CEAFAR panels and you'd be in business for self defence.
I don't know why the modular thing is such a shock to any one because the people who pioneered this, the Danes with the STANFLEX ships, tend to leave the ships in the same configuration for long periods simply to allow the crews to stabilise around a role and become proficient in it.
Think of the modular aspects as being flexibility to upgrade and repurpose ships during refits and they look more sensible.