How to differentiate EVENT_BEFORE_UPDATE_SUBSCRIPTION events in Commerce 2/3
Could you use one of the following events?
\craft\commerce\stripe\services\Invoices::EVENT_BEFORE_SAVE_INVOICE
\craft\commerce\stripe\services\Invoices::EVENT_SAVE_INVOICE

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 that comes into commerce, and react.
Please comment on the ticket for that here if so: https://github.com/craftcms/commerce/issues/1799