  Nontrivial dark sector physics continues to be an interesting avenue in our
quest to the nature of dark matter. In this paper, we study the cosmological
signatures of mass-varying dark matter where its mass changes from zero to a
nonzero value in the early Universe. We compute the changes in various
observables, such as, the linear matter power spectrum and the cosmic microwave
background anisotropy power spectrum. We explain the origin of the effects and
point out a qualitative similarity between this model and a warm dark matter
cosmology with no sudden mass transition. Finally, we do a simple analytical
study to estimate the constraint on the parameters of this model from the
Lyman-$\alpha$ forest data.
