I believe that the numbering system will still be the same, on the basis that the berths and seats on the existing stock was renumbered recently so as to reflect the (now delayed) introduction of the new Mk V stock.
For example, in the present Mk III stock, the twin/shared berths used to be numbered from 1 to 24 (or 26), with the odd numbers being for the lower bunk, and even numbers for the top bunk. The berths were recently renumbered from 1 to 12 (or 13), with a "L" for the lower bunk, and a "U" for the upper bunk. On the new Mk V stock, I think that the "L" and "U" designations will be removed 9as will the ability to share with a member of the same sex), as tickets sold will be on a per room basis rather than per bed.