PayPal subscriptions with promotions

[shoelaced]

If I offer a promotion for a recurring service, such as a discount for a certain number of months, and the client pays the initial invoice by creating a PayPal Subscription, will the subscription amount be auto-updated after the promotion?

If not, then how does WHMCS handle this situation by default? Does it cancel the subscription automatically and re-issue a new invoice through which the client has to manually create a new subscription? Or does it just let the subscription continue, in which case the client is presumably going to end up with half-paid invoices in their account? If that happens, how does the client pay? If they pay the remainder of the invoice as a subscription, wouldn't they then end up with two subscriptions? Does the client have the ability to cancel or edit their subscription through WHMCS?

I can see not auto-updating the subscription, otherwise anyone could randomly increase the price after the client created a subscription, but if the client pre-agrees to the post-promotion price at the time of purchase, then the subscription should be allowed to auto-update for that price only. If that's not possible then it should gracefully handle it automatically by auto-canceling the subscription, auto-creating a new invoice for the full amount, and auto-emailing the client to say "Your promotion has ended. To prevent interruptions in service, please log in and pay your unpaid invoice(s)," or something along those lines. I can't imagine our team having to manually cancel everyone's subscriptions, what a huge pain...