Eh, people in Joss's shows change hair styles all the time.
And remember how well he hide his feelings for Kaylee. It wasn't unti lthe film he admited sharing them.
Didn't Joss say he planned five or sevens seasons of the show? I'm sure he had paths and ideas. "Serenity"
may have been something like a season 2 or 3 finale that got built up to.