I would agree, most of the bacteria are on your filter sponge anyway, so changing the substrate, especially one thats large gravel like this shouldn't be a problem. But yes wait another month or so to get things settled.
I like it I think you`ve done well just the gravel isn`t my favourite I think a more natural looking tank is always better but I haven`t even got a tank yet , I`m hoping to get a Rio 125 do you recommend it ?