The injection of green hydrogen and biomethane is currently seen as the next step towards the decarbonization of the gas sector in several countries. However, the introduction of these gases in existent infrastructure has energetic, material, and operational implications that should be carefully looked at. In this paper, developed with Galp Gás Natural Distribuição, impacts on gas transmission and distribution are evaluated and an interchangeability analysis, supported by one-dimensional Cantera simulations, is conducted. It is concluded that hydrogen content up to 20% by volume is possible to accommodate in current infrastructure with only minor technical modifications. However, the different tolerances for hydrogen blending of consumers, depending on end-use equipment, may be critical during the transition period to a 100% green gas grid as there is a risk of pushing consumers off the grid.