Given a complex $M^{\bullet}$ (say in an abelian category, in particular a module category) with differential $d^{i}: M^{i} \rightarrow M^{i+1}$, we can form another complex $\widetilde{M}^{\bullet}$ with differential $\tilde{d}^{i}=(-1)^{i}d^{i}$.
How are these complexes related?
They certainly have the same cohomology (since the signs don't change kernels and cokernels), so that suggests that they could be quasi-isomorphic, but I don't see how to write down the quasi-isomorphism. The identity is not a cochain map (outside of characteristic $2$), nor is alternating the signs on the identity.