% \change Staff = "lh" doesn't work without \key c \major.

\version "2.26.0"

rh = \relative c' {
%  \key c \major 
  \change Staff = "lh"
  c
}

lh = \relative c {
  \clef bass
  c
}

\new PianoStaff
<<
  \new Staff = "rh" \rh
  \new Staff = "lh" \lh
>>