How come in the 1st Star Wars triogy, Yoda was Obi-Wan's master but in the 2nd trilogy, it's Qui-Gon Jinn?

In the original trilogy Yoda was Obi-Wans master (said so by Obi-Wan
himself and Yoda) but in the prequel, it's Qui-Gon Jinn? Am I
mishearing something or dude George Lucas had a different mindset at
the time...?
But I thought as soon as Qui-Gon died, that's when Obi-Wan took in
Anakin? (In the Phantom Menace)