Elstree

1) I have an 18 digit reference number beginning 6099999 which I have used in previous years paying backdated contributions for more than 1 year. This year I only have to pay for one year i.e. 2022/2023 for which I will owe £824.20. Do I only need to change the beginning of the reference number to 6082420 and the rest of the number remains the same? Do I wait until the end of this tax year to make this payment i.e. 5th April 2023 or can I pay it now as my HMRC account national insurance record for this period says "your record for this year is not available yet"? 2) The amount for 2023/2024 is now set at £907.40 as per the recent budget - when is the earliest I can make this contribution or could it change during the year? Thank you for your help.