Direct debit date change
@Anoush wrote:
Being able to change your billing date is still a work in progress that should come with a brand new billing system. We currently don't know when it's expected to be implemented I'm afraid.
These words keep buzzing around in my head - just can't remember the tune?
Currently 5+ years and still in gestation 
Still working on that "new" billing system?
When it arrives, you will be able to call it the old billing system. 
Welcome to the community forums @MikDundee.
I'm sorry to hear you're thinking of leaving due to not being able to change your billing date. As I don't want to set any incorrect expectations regarding when this functionality can be expected, there is unfortunately nothing I can advise further to Anoush's post back in November.
If you were to start a new account, the billing date would be set as the day the first service goes active, however I wouldn't be able to give a 100% guarantee that you'd still get the requested date as orders wouldn't complete over weekends and there is always a small chance that a delay could occur on the order.
An averagely flexible company would be able to take a part month's payment, followed by an amended direct debit date.
I bet the new system won't be able to help me, I get paid four-weekly. 
Switch to a company that has more flexible arrangements: https://aa.net.uk/legal-billing.html
We have a sophisticated billing system which allows several customer options. Some of these may be selected when setting up an account. Some require you to contact our accounts department to change them.
The preferred billing option monthly Direct Debit.
Periodic billing
Some services are provided on an ongoing basis. For these you are billed periodically, normally in advance on the first day of the period covered by the bill. We can insist on calendar monthly bills if we wish, but this would normally be if we are having problems with your payments being made on-time. We can also invoice you several days before the period starts.
The following billing periods are available. They relate to all services on the account. We recommend monthly.
- Calendar monthly: Invoices are for the whole month from the 1st of each calendar month.
- Four Weekly: Invoices cover a 4 week period, the exact starting week can be adjusted if required (e.g. to align with salary or benefit payments arriving). Prices are based on 23% of our monthly rates for each week billed.
- Lunar monthly: Invoices are issued each full moon for the period to the day before the next full moon. Prices are based on 97% of our monthly rate for each lunar cycle. Yes, we really do this and we really have customers that use it.
- Quarterly: Invoices are for Jan-Mar, Apr-Jun, Jul-Sep, and Oct-Dec. We can adjust which month the quarter starts on if necessary. See note below.
- Annually: Invoices are for the whole year from 1st Jan. We do not normally provide annual billing. We can adjust which month the year starts on if necessary. See note below.
Partial period bills
When a service starts or ends your bill may be for a partial period, e.g. 1.5 months, so as to align to the selected billing cycle. When working out how many months a period is, we work on whole days. Part days count as a whole day. For billing cycles based on calendar months, each day is considered as a part of its month - i.e. 1/28th or 1/29th of February. For billing based on weeks, each day or part is 1/7th of a week. For lunar billing, lunar months are considered 29 or 30 days based the full moon being the first day of the lunar month, and so each day is 1/29th or 1/30th of its lunar cycle.
I'd like to try the lunar option. 