The only way I can think to do this is with a calculated field.
There is a function called Substr() that will check position 5 (or whatever position you need) in the string, and another function called Stuff() which allows you to replace only what's in a specified position. The total formula would look like this:
If( Substr([Employee ID],5,1) = "7", Stuff([Employee ID],5,1,"Replacement String"), [Employee ID] )
Keep in mind, these functions only work on strings, so if that field is numeric, you will need to convert it to text prior to entering this formula.
If you need more explanation, let me know!