Did they add CGI to the older Star Wars films?

I'm watching Return of the Jedi on Spike right now, and I'm not an aficionado so I don't remember everything. But during the scene where the band is playing for Jaba the Hut there is a singer that looks like a muppet and he looks CG'ed. Also, when they flew over the sand dunes to the pit monster the crafts looked "computer fake". Did Lucas tinker with these scenes?