A much better mental model of this stuff is Newtonian mechanics is always wrong at every scale. At human masses, velocity's and time scales you can't detect the error but it's still there.
As such the physical laws are unchanged at the big bang, but the simpler aproximations in current use may be noticeably off at these scales. If they are, then they would also be wrong at all other timescales and energy levels etc. The trick is looking for edge cases we can detect that seperate our aproximations from the actual laws running reality.
Given all that; simpler aproximations continue to be useful as long as the errors are hard to detect.
As such the physical laws are unchanged at the big bang, but the simpler aproximations in current use may be noticeably off at these scales. If they are, then they would also be wrong at all other timescales and energy levels etc. The trick is looking for edge cases we can detect that seperate our aproximations from the actual laws running reality.
Given all that; simpler aproximations continue to be useful as long as the errors are hard to detect.