True. I've never understood that. Even with the OT he had differences (compare ANH Jabba compared to ROTJ Jabba).
dies ist meine unterschrift

The changes in tone are understandable, such as the ships and everything starting to look more dire and militarized.
But the overall change of aesthetic goes beyond that. A good example is the Yoda puppet they originally used. Aesthetically, it looks totally different from the CGI yoda that replaced it. Clearly, Lucas started off with a very different idea of the look he wanted for the prequels.
Another example is the cinematography during the Jedi duel, the camera frequently switching to different vantage points, distances and angles. It was much less pronounced in the sequels.
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According to an interview with Lucas, he did that on purpose. It shows the effects that the Sith and the Dark Side had on the galaxy. In TFM the Sith weren't in power, everything was bright, shiny and clear. As the movies go on and Palpatine is in power everything starts getting darker, grittier. By the time the Empire is established in ANH the galaxy is much darker, everything is run down and broken. It's the effects of the shadow of the Dark Side.
dies ist meine unterschrift

it's kind of ironic that the practical effects are so good people think they were CGI. (as if cgi could be that good in 1999)
"He's dusted, busted and disgusted, but he's ok"

Doug Chiang was replaced by medicore church and had less and less freedom as the trilogy went by
Lucas also throw away a winning formula of phantom menaces mixture of practical and cgi for a more cgi dependent film

Lucas became completely obsessed with CGI.