We prove using an integral criterion the existence and completeness of the wave operators $W_{\pm}(Δ_h^{(k)}, Δ_g^{(k)}, I_{g,h}^{(k)})$ corresponding to the Hodge Laplacians $Δ_ν^{(k)}$ acting on differential $k$-forms, for $ν\in\{g,h\}$, induced by two quasi-isometric Riemannian metrics $g$ and $h$ on a complete open smooth manifold $M$. In particular, this result provides a criterion for the absolutely continuous spectra $σ_{\mathrm{ac}}(Δ_g^{(k)}) = σ_{\mathrm{ac}}(Δ_h^{(k)})$ of $Δ_ν^{(k)}$ to coincide. The proof is based on gradient estimates obtained by probabilistic Bismut-type formulae for the heat semigroup defined by spectral calculus. By these localised formulae, the integral criterion requires local curvature bounds and some upper local control on the heat kernel acting on functions provided the Weitzenböck curvature endomorphism is in the Kato class, but no control on the injectivity radii. A consequence is a stability result of the absolutely continuous spectrum under a Ricci flow. As an application we concentrate on the important case of conformal perturbations.