If that change is made alone, this will break some contract for sure. We for example use the gas opcode to ensure a meta-tx receive the proper amount of gas. If the gas opcode always return 2**156-1 then our contract will always assume the meta-tx received enough amount of gas while it might not be the case. Relayer could thus maliciously make the meta-tx fails while getting the reward, if any.
I have been pushing for an EIP for quite some time that would remove the need for the gas opcode for taht particular use case (metatx)
So if we could get this first : ERC-1930 Allows specification of a strict amount of gas for calls
Then maybe we could deprecate the GAS opcode
But the problem still remain that existing contracts will be affected