How long ago did you buy Pro? Your Pro purchase covers you for 6 months worth of updates. If you bought Pro less than 6 months ago, just go to your personal download page & download 4.12.3 If you didn't keep the link to your personal download page, go here & enter the email address that you used to buy Pro
http://sales.limewire.com/support/pro_lookup.php

If your Pro has expired, did you keep the 4.10.9 Pro installer? Installing a new version of LW will remove the old version, but if you still have the Pro installer you can reinstall it any time If you didn't keep the installer then you'll lose your present version if you install 4.12.3.