How to change payscale level for the previous date for an employee.

Hi Experts...

Some how my user has wrongly changed the Grade of an employee from 01.01.2006.Actually he has to be in A3 Grade.Some how it has changed to A4 grade even though

now also we are paying him as per A3 grade only.How to change the grade of that particular employee from 01.01.2006 onwards.Please advice.

Regards,

Sai.