I have a Visual Basic PCL for Xamarin. Will I need to make any changes to the PCL to be able to use it in a Xamarin Unified API project? Or just recompile perhaps?
I read that Visual Basic code requires no changes for 64 bits unless you are using the fact that it's 64 bits, which I am not.
You don't need to do any changes to PCL projects. The only projects who needs updating is iOS and Mac. For more information on updating that see the documentation: http://developer.xamarin.com/guides/cross-platform/macios/updating_ios_apps/