cant you serve it centrally and then just make updates on the core production instance
you would be running a subscription based business model
clients just need to log in to your system and not worry about updates
This is how I would have done it but you have said the product is already being sold now so your too late I suppose
The only other way would be for your script to check a file in a directory on your server and check the build number of that file. If its different from the one it has registered previously then you need to download it and then replace it.
Its not a very good implementation though.