Poser began this irritating behaviour with 4, and it has not been fixed yet. Hopefully it will be gone in Poser 6. Basically, what happens is that it uses a faulty naming system for new lights vs old ones, and when you attempt to load one type over the other, the lights already in the scene get disabled, and new, shadowless lights are added. One workaround is to add as many or more new lights to the scene prior to loading from the library. This will ensure that only pre-existing lights, which have shadowcams, are used. Unfortunately, this will leave you with a bit of a mess if you decide to adjust any of the lights afterward.