Question about UOM and Availability

I have an item where the primary UOM is EA and we have a conversion set up to be able to sell it by the case (1 CS = 4 EA). 

For this particular item, we only have 3 EA available, so not enough for a complete case. 

When we enter an order for 1 CS, the order goes on backorder, as expected.

In the event that we try to create an order for 1 CS and there are are only 3 available, is there any way to have the UOM change to the quantity and UOM that is available? In this case, the order for 1 CS would change to one for 3 EA, since there are 3 EA available, but not enough to make a full case.