Hello AmandaBenedict4,
Welcome to the community!
In general, it is not usually possible to change your billing cycle. Your billing cycle is determined when you activated your services (usually starts a few days after service activation) and does not change, even if you change your services. However, it may be possible to do so under exceptional circumstances. You should note, though, that there may be additional charges for having it changed (see here). As well, you may not be able to keep your current plan and may need to pay-off any current balance on a financed device. As far as I am aware, I think the reason is due to the need to cancel services and then re-start services with a new billing cycle.
I understand that you have described changing the payment due date, and not necessarily the billing cycle. However, the payment due date is directly related to the billing cycle. The payment due date is now 21 days after the bill date (see here). It would not be possible to change the payment due date without changing the billing cycle.
You should note these forums are community-driven and are not intended as a venue for customer support. We would not know the full realm of what may or may not be possible. If you wished to enquire futher regarding changing your billing cycle, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
