I think the only thing you should update for this would be the action of the form the button is in. The action should be going to the sandbox instead using this value:
I got this from the PayPal Sandbox Users guide on page 34. Here is a link to the page that has the download for this documentation:
Just click on the Sandbox Users Guide link to download it.
Normally when testing with the Sandbox you are just adjusting the gateway or location that the information is being posted to.