Could you use one of the following events?
Maybe looking in the invoice model invoiceData attribute (that is passed to the above event).
If that won't suffice, we should expose a way for you to listen to any Stripe Webhook event ...
You're probably looking for Element::getDirtyAttributes() and Element::getDirtyFields()!
The former is for native class properties/attributes (and is based on an ActiveRecord pattern in Yii, I believe...), and the latter is for Craft's "fields," which may or may not be available, based on what type of Element you're looking at (and what FieldLayout ...