  We present results of the first lattice QCD calculations of $B_c \to B_s$ and
$B_c \to B_d$ weak matrix elements. Results are derived from correlation
functions computed on MILC Collaboration gauge configurations with lattice
spacings between $0.12$ [fm] and $0.06$ [fm] including 2+1+1 flavours of
dynamical sea quarks in the Highly Improved Staggered Quark (HISQ) formalism.
Form factors across the entire physical $q^2$ range are then extracted and
extrapolated to the physical-continuum limit. Two different formalisms are
employed for the bottom quark: non-relativistic QCD (NRQCD) and heavy-HISQ.
Checking agreement between these two approaches is an important test of our
strategies for heavy quarks on the lattice.
