The time between Xiao Guo going oh no she’s dead and her transforming is so small, it’s impossible to tell if changing forms was still her own doing. It didn’t read to me like it was – I can’t really back this up, though, it’s just how the scene felt to me.

Me too. I do think we're supposed to read it that way.

Maybe it's to do with what proportion of their lives they spend in either form -- ie, Dong Nan grew up largely as a Crow and only spent a small amount of time in human form, so she reverted to Crow at death, whereas Da Ji had been running around on two legs a lot and stayed that way when she died? /random theory