Due to the increasing IC design complexity, Engineering Change Orders (ECOs) have become a necessary technique to resolve late-found functional and/or timing deficiencies. To fix timing violations, the principles of gate sizing and buffer insertion are commonly used in post-mask ECO. These techniques however may not be powerful enough, especially when spare cells are inserted in a way of striking a balance between functional and timing repair capabilities. We propose a post-mask ECO technique, called TRECO, to remedy timing violations based on technology remapping, which supports functional ECO as well. Unlike conventional technology mapping, TRECO performs technology mapping with respect to a limited set of spare cells and confronts dynamic changes of wiring cost incurred by different spare-cell selections. With a pre-computed lookup table of representative circuit templates, TRECO iteratively performs technology remapping to restructure timing critical sub-circuits until no timing violation remains. Experimental results on five industrial designs show the effectiveness of TRECO in ECO timing optimization.