Generating key values for leader records

All,
I have a file with text as shown below. I want the o/p file with generated values in the first column as shown in the o/p file. Pls note that the size of my file is 6 GB. How do i do this ?

Input file
999999abcdef
999999ghijkl
999999mnopq
777777rosesarered
777777skyisblue

Output file
1999999abcdef
1999999ghijkl
1999999mnopq
2777777rosesarered
2777777skyisblue

Hi,

Do you want the value 999999 to be replaced with 1222222 and
777777 to be replaced with 2333333 ?