Preventable loss can cost much more than testing. The most serious problem is the wish to test only for errors that stop a system. Not testing for logic bugs may impact accuracy or performance of downstream programs or data bases. They may only surface after doing considerable damage. The belief that instant failure is the sole risk must be reversed now to avoid business loss and higher renovation costs later.
Emphasis is on date testing.
This neglects many tests that could locate remaining errors. Testing should be observed by an independent audit team. This reveals many errors caused by change. It may be added to other categories of test. Copies of original programs must be saved.