You can always update to the latest basic for free, but this will delete the current pro version you have (which one is it by the way?... you can find out in About Limewire under help in windows.)
You may want to keep a copy of your old verison saved to disk or somewhere safe, or the installer if you have it, as you may decide you want to go back to it.
If you stay as you are you will have to live with the reminder I'm afraid, but you do not need to follow it's advise
or you can pay $18.88 for the latest pro which is not a lot for a fine product. It's up to you.