First of all, someone needs to verify that large quantities of hydrogen exist underground. The best of all worlds would be if it existed in large quantities in gaseous form and would be as easy to extract as natural gas. Somehow, I doubt that's the case. If it's trapped in rock as claimed in this sentence ("Professor Freund said that his team had "tantalizing evidence" that as much as 1,000 litres of hydrogen may be trapped in each cubic metre of rock."), then things become a lot more difficult. How much energy would be required to extract the hydrogen? Energy costs money, so the harder the hydrogen is to extract, the less cost competitive the new sources would be. That's one of the reasons why shale oil never was cost competitive with petroleum - it just took too much energy and water to extract the oil from the shale.

Converting to a hydrogen infrastructure would be very costly even if we could find a cheap underground source for the gas. We'd probably need new pipelines and storage facilities due to hydrogen's chemical properties. We'd need new tanker trucks and fuel pumps, too. We'd need new fuel tanks and engine modifications for vehicles, similar to the changes necessary to convert a car to burn propane.

All of these things are possible from an engineering perspective but each costs quite a bit. There are over 100 million cars in the US alone. It'd probably cost at least a thousand dollars per vehicle (and probably quite a bit more) to convert them to hydrogen power. That's $100+ billion right there and we have not even done anything else to convert gas stations, etc.