It would depend on the client’s requirements. In many cases, the client may specifically require you to re-execute the entire verification and validation protocals. This can be costly but if the client does not receive the expected results, that would mean loss of money and future deals. So the best way to handle this situation would be to see if the final result is what the client wants. If this can be fixed by regression test, than it would cost less and you are able to deliver the expected results. However, if you want to save the time and cost by taking a short cut of a regression test and the final result is not what was expected, you end up with more cost and time to your company. So in many cases, it is better to speak with the direct clients to avoid the loss of time and cost.