Query for converting a column of numbers matching a regex in MySQL

I have a column which contains string values of the format e.g AB12345678 - two letters followed by 8 numeric digits. I need to write a MySQL query to insert a 5 before each set of 4 digits thus AB12345678 becomes AB5123455678. I'm close, the following works for one value in the column, but not for multiple values - all values are written with the same first value encountered it seems.

SELECT @twoeight := os_grid_ref FROM projects WHERE os_grid_ref REGEXP '[A-Z]{2}[0-9]{4}[0-9]{4}';
SELECT @prefix1 := SUBSTR(@twoeight,1,2);
SELECT @part1 := SUBSTR(@twoeight,3,4);
SELECT @part2 := SUBSTR(@twoeight,7,4);
UPDATE projects SET os_grid_ref=CONCAT(@prefix1,'5',@part1,'5',@part2) WHERE os_grid_ref REGEXP '[A-Z]{2}[0-9]{4}[0-9]{4}';

What do I need to adjust so that the query can modify each value in the column individually. Thank you.