Do this in on a test server before you apply it to production. In the query below, you can leave the red KeyCode as it is if you just want any random 6300 records to get one pack code. But if you want to order it by some criteria, so that the first 6300 would be the first in that ordering scheme, change the red Keycode to whatever columns you want to order it by
;WITH cte AS
ROW_NUMBER() OVER (ORDER BY KeyCode) AS NN,
KeyCode = 'AX10'
UPDATE cte SET
PackCode = CASE WHEN NN <= 3700 THEN 'AX10002' ELSE 'AX10012' END;