The changing of the bolts is a safety issue on a critical application.
Basic theory is : When a bolt is tightened to the max, it will stretch a fraction.
When the bolt is removed it will stay stretched
If the old bolt is then re-used it is likely to stretch again.
This in theory may result in a weaker bolt that is just within its yeild zone & in theory is more likely to fail ( Shear )
More science than that involved but that is the basic theory
