Any particular sources? From a quick web search, it sounds like hydrogen has a very different energy density (Joules/cu. m., or BTU/cu. ft., or whatever), very different combustion and leak safety characteristics, and can either leak through or embrittle many parts of the existing (natural gas) distribution infrastructure.

If that's anything resembling correct...then the change-over would be more like replacing all-diesel fuel & engine infrastructure with gasoline.