An $n\times n$ circulant matrix $A$ with all entries nonzero is known to generate an MDS matrix. Here $n$ is 4, with alphabet $GF(2^8).$
An MDS matrix guarantees maximum diffusion in the MixColumns step. See the answer to the following question how-to-calculate-active-s-boxes-from-branch-number on why the fact that the minimum (symbol) Hamming weight achieving the maximum possible value of $n+1$ is important.
As pointed out by @Amin235 the new matrix you obtain is still MDS, but not being circulant, you've sacrificed computational speed. Since all MDS codes have the same weight distribution, not only the minimum possible diffusion, but all diffusion properties for all input/output vectors are identical between your new matrix and the original AES MDS matrix.