Changing AR Invoice Line details during AR Invoice Entry

We just updated from 8.03.403d to 409c and are now getting an error we didn't used to get.

In AR Invoice entry, After selecting a packer using "Get Shipments", we get an error if we change any of the invoice line deatils (product group, unit price, customer P/N, etc...).

It looks like the line from the original Order needs to have the "Time/Materials Billing" checkbox set in order to make changes to the details. But this wasn't required before, and the editable fields are disabled when the T&M flag isn't set.

What to others do when the invoice details need changing?

Thanks,

Calvin