The interaction between zinc(II) tetraphenylporphyrin (ZnTPP) molecules and the Ag(100) and Ag(111) surfaces is investigated using a combination of scanning tunneling microscopy as a local probe of the molecular adsorption configuration and x-ray, ultraviolet and inverse photoemissions as probes of the electronic structure. For each surface, a monolayer of ZnTPP, formed by multilayer desorption, exhibits a highly ordered structure in registry with the underlying surface lattice. Subsequent annealing leads to a transition from intact molecular adsorption to dehydrogenation and subsequent rehybridization. This rehybridization is both intramolecular, with a flattening of the molecules and a measurable alteration of the electronic structure, and intermolecular leading to 2D-growth of extended covalently bound structures.