When the Law was given to Moses it was refined from the simple commandments given before and immediately after the flood. The Law clearly laid out what meant what, with all ifs, ands or buts covered.
Take rule one for example.
1.) If a man sheds another mans blood, then by man shall his blood be shed.
Very simple and basic. However in the Law of Moses you find many examples of where the rule is refined. Cities of Refuge and the difference between Homicide and Involuntary Manslaughter being just two examples.
2.) Every living creature I have given to you as meat but the blood that is the life you shall not eat.
Rule two has many of the same refinements.
Prior to the flood there were clean and unclean animals but the rule had nothing to do with eating the animal. Another refinement, if you will. Arbitrary? No. Refined? Yes.
A. Cricket
We are not talking about the nature of God changing. We are talking about the nature of animals changing.
I find nothing scriptural that details what is clean and unclean prior to the flood. Seems that the only command was that quoted by AC earlier. Every beast is for food.
GSA(P)