If you already own Skyrim on PC and have invested in all of the DLC, you won't need to shell out for the upcoming, upgraded special edition. The same thing is true if you own the Legendary Edition.
Bethesda has announced that anyone who has already bought into the full Skyrim experience will be taken care of. The special edition will be given to those PC customers free when it's released.
What we aren't clear on quite yet is how existing mods will function with the newer version. The PC version features a huge number of mods, including changes to graphics, sound, and user interface. We'll update once we have a better sense of what kind of work mod creators will need to do (if any) in order to get their creations compatible with the special edition, coming on October 28.