1. Do not shut down old versions of Skype. Make Skype updates optional. Users get used to their "old" versions and often DON'T WANT any changes in user interface. Remember that Skype is often used by beginner users (for example, parents and grandparents) and for some of them it is a big pain when familiar user interface got changed because of some "bright" ideas of UI designers. Remember that such users will often not need your new features also.
2. If your marketing campaign and product strategy are still more important than how many users feel about your products, make Skype updates AUTOMATIC as it used to be before Skype was purchased by Microsoft. Let's not go back to previous century when users had to update all software manually.