We report the use of localized annealing via in situ heaters to induce a semi-permanent change in the refractive index of the cladding in ring resonator filters. When compared to other methods for post-fabrication trimming, this method has the advantage that no additional equipment, other than a supply of electrical power, is necessary to cause the index change. Two cladding materials were used: hydrogen silsesquioxane (HSQ) for samples that were externally annealed, and PECVD oxide for samples that were annealed with in situ heaters. The resonant wavelengths could be adjusted by as much as 3.0 nm and 1.7 nm for the HSQ and PECVD cladded filters, respectively. The trimming of a 5 channel, single ring filter bank, and a single, double ring filter is demonstrated.