Heres a list of documentaries i made (loosely) of a niche genre. They are all documentaries that started about one thing but ended up being about something else.
9/11 (2001)
American Movie
An Honest Liar
As the Palaces Burn
Calabasas 1/26/20
Capturing the Friedmans
Capturing the Light
Citizenfour
Collective
Dave Not Coming Back
Dear Zachary
Dig! It
Fairytale of Kathmandu
Gimme Shelter
Grizzly Man
Hearts of Darkness
Hoop Dreams
Hurricane on the Bayou
I am Trying to Break Your Heart
Icarus
In the Jinx
Into the Deep
Into the Inferno
Life With Murder
Lost in La Mancha
Marjoe
Minding the Gap
My Kid Could Paint That
Operation Odessa
Overnight
Rock and Roll Nerd
Sasquatch
Savile
Searching for Sugarman
Seaspiracy
Shemans March (1985)
Sherpa
Stevies
The Estonia
The Hamiltons
The Imposter
The Jinx
The Queen of Versailles
The Revolution Will Not Be Televised
The Sweatbox
The Thin Blue Line
The Woman Who Wasn't There
There Are No Fakes
Tickled
Vernon, Florida
Weiner
Wrestling With Shadows
Have toucan, can confirm.