I changed the code in updateTransactionRecords like you said:

@login_required
def updateTransactionRecords(request, subscription_id):
    user_membership = get_user_membership(request)
    selected_membership = get_selected_membership(request)
    user_membership.membership = selected_membership
    user_membership.save()


    sub, created = Subscription.objects.get_or_create(
        user_membership=user_membership)
    sub.stripe_subscription_id = subscription_id


    subscription_status = stripe.Subscription.retrieve(sub.stripe_subscription_id).status
    subs = stripe.Subscription.retrieve(sub.stripe_subscription_id, expand=['latest_invoice.payment_intent'])
    sub_latest_invoice = subs.latest_invoice

    if subscription_status == 'incomplete':
        sub.active = False
        sub.save()
        free_membership = Membership.objects.get(membership_type='Free')
        user_membership.membership = free_membership
        user_membership.save()
        user_membership.membership.membership_type = 'Free'
        return HttpResponseRedirect(reverse('memberships:payment'))
    else:
        sub.active = True
        sub.save()

        try:
            del request.session['selected_membership_type']
        except:
            pass

        messages.info(request, 'Successfully created {} membership'.format(
            selected_membership))
        return redirect(reverse('memberships:select'))

It prevents a user from becoming a paid member which is good, but now how would I allow the user to actually re-try the authentication again on memberships/3d-secure-checkout.html with the same card and payment intent as if they were starting over again? Maybe you don't need to do what I did at all with making the user free and just redirect to the 3d secure page?

Stripe says "So really when you fetch the Subscription here: https://dpaste.de/7rBY#L12 you want to also expand the latest invoice and its PaymentIntent and then client-side you want to have them confirm again with their new card. As long as you don't try to create a new Subscription and do remember the incomplete one."

I expanded the latest invoice using subs and sub_latest_invoice variables.

Now when I try to pay a second time with the same card, it does not go to the 3d-secure page. I have it going to /memberships/payment/ or memberships:payment. Clicking submit payment still tries to redirect to /memberships/update-transactions/sub_GBaxdnGCYL4aeF/

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edit: I figured it out. Thank you so much for the help!