Primary Key Modification.

Hello,
Have a huge employee table with employee # set as primary key starting with letters A-Z plus 4 digit # example B7865.
Now I would like to replace A-Z with 01-26 but also keeping the old record. Example if old record is B7865 updated one would be 027685 and Z8767 would be 268767.
Any ideas how can this be done.
NB: this table has more than 90k records so it must be an efficient way of searching.

Thanks.