BOM editing during serial production

We have the following situation:

We have created 10 x Manufacturing orders of Product A with BOM A.

We manufacture 2 of those 10 products according to BOM A and realize that we have a problem within this BOM and need to change it (e.g. product has been discontinued and is no longer delivered)

Therefore we want to create a revised BOM B – at this point we want to also change the BOM of the 8 Manufacturing orders that are still open.

We are manufacturing products  by "make-to-order". 

If I just change the BOM from A to B it does not effect the remaining 8 orders. The only way I can do it now is cancel them and start them again. 

Please help me out here :)