The billing date for business cards seems to be 22 days before the due date. April 9 due date implies March 18 closing date. Verify with customer service if the exact date is critical.
This can be done using the self-service function on the website. No need to send an SM. However, be aware that the change does not become effective during the current billing cycle.
Yeah, business cards have shorter grace period than personal cards IIRC.
You can only request a change on closing date every 6 months I believe.