Well, the Pro version offers updates and tech. support for 6 months. After that you've still got the Pro version, just without the two thing said above.
Usually, Pro buyers should save their installer .exe somewhere save in case something unexpected happens, so they are able to re-insll their paid version.
If you're still in the 6 month limit, you could go to your download page and re-download the installer exe. However, if you aren't, you'll have no alternative then to download the newest basic version or to purchase the newest Pro version. Of course, either you should do only from the official LW website:
http://www.limewire.com/