add a body class when in switch subscription mode
We think the checkout process for changing subscriptions is super cumbersome*. Something that would make it easier to customize would be adding a body class like "switch-sub" so we could target specific elements, to hide what isn't relevant. (The class could also be applied to the same line where the product ID is written.)
*The whole checkout process doesn't apply in our context where most customers need to simply change a product variation to be shipped monthly.