Hi,
If your circumstances have not changed since you first applied to pay voluntary National Insurance contributions (NICs), the class of NICs will not change following your request to change your method of payment from an annual payment to direct debit.
If any of your circumstances have changed which would affect your eligibility to pay voluntary NICs at the class 2 rate, for example if you are no longer employed or self-employed abroad, or you no longer meet the criteria shown in the NI38 -
Social Security abroad: NI38) you should contact HMRC to inform them of your change in circumstances.
With regards to making a payment via bank transfer using the reference number you have quoted in your query, this is for payment of previous tax years, and it is not an option for future ongoing payments.
For future tax years, you can either pay by Annual payment or monthly/6 monthly direct debit.
Unfortunately, it is not possible to receive a digital letter for an annual payment.
Thank you.