It can happen....just not often,
EG: 1977, WHen he changed the title of "Super happy Jawa adventure" to "Star wars"
1979: Agreeing to let someone do the star wars holiday special.
1986: Howard the duck. 'Nuff said.
1999: George changes his mind about wanting to do quality starwars prequels, so he revises the plot, wastes the opportunity to start in AOTC times, sticks in JarJar
2007: KOTCS giving in on the gopher flying the jetpack idea AND the bear skeleton in the ark that breathes force shields.