Harmy said:

I think it might actually be an intentional change. Maybe they were trying to conceal the AT-ATs' feet being hazy by making it appear that there was actually a misty haze on the ground level, because it does look a little less like the AT-ATs are floating in the air.

And the disgustingly smeared and burnt "detail" on the foreground figures? I think you're being too generous by assuming they had a specific goal in mind. ...and I for one never thought there was any problem with the AT-AT/ground interaction.