No good solution for you but I am somewhat surprised this is an issue. When you change the customer number, you are asked if you would like to update the pricing.... If you answer Yes, one would think the program would also update the GL account numbers.... they are already making a pass through the line items.
How many companies found out the hard way about this lack of a feature? There is no warning, you just happen to notice at the end of the month that you need to reclassify 400 transactions. Good grief.