blackbirdgunner

[unkempt perch]

we only want to get them on upgraded plan if their payment was successful
That's expected yes:
https://stripe.com/docs/billing/subscriptions/upgrade-downgrade#:~:text=When billing is performed immediately%2C but the required payment fails%2C the subscription change request succeeds and the subscription transitions to past_due.

What you can do is to listen to the failure webhook event, and downgrade the subscripion by your integration in that case.

[lyric sphinx]

is there any particular failure webhook event I should be listening to for this? Will invoice.payment_failed work?

[unkempt perch]

yes that works

[lyric sphinx]

Would I also need to void the new invoice that was generated or will downgrading their subscription using stripe.Subscription.modify() take care of that?

[unkempt perch]

You'll need to void the generated invoice too manually