  Small changes in the masses of massive external scattering states should
correspond to small changes in the non-perturbative parameterization of form
factors in quantum field theory, as long as the relevant energy range is not
near strong deformations. Here, the definition of ``small'' is investigated and
applied to $SU(3)$ breaking in semileptonic $B_{(s)}\rightarrow D_{(s)}$
transitions. When unitarity and analyticity are imposed, the differences in the
form factors for semileptonic $B\rightarrow D$ versus $B_s\rightarrow D_s$
decays are found to be within $\mathcal{O}(1\%)$ over the entire kinematic
range, not just at zero recoil, which is consistent with results from lattice
calculations and differs from the expectation using HQET alone.
