[tlocke]

To take the USA for example, it'll play out like this. As more and more renewables are built there will be periods where the amount of renewable electricity will exceed demand. During these periods, electricity will be very cheap and will be used to create hydrogen. The existing natural gas storage and pipework will be used for hydrogen instead, and hydrogen turbines / fuel cells will be built. So the natural gas infrastructure will be taken over by hydrogen, generated from renewable electricity. This gradual takeover will extend to heating. Natural gas boilers in homes / buildings will be converted to hydrogen, and so in the end the USA will end up with zero carbon emissions for electricity and heating.

[DennisP]

Sounds good, but has anyone done this in production anywhere, and if so, what did it cost?

Hydrogen has a habit of leaking through anything and embrittling metals, so it may not be a trivial task. According to the Dept. of Energy, converting pipelines to carry pure hydrogen will require "substantial modification."

https://www.energy.gov/eere/fuelcells/hydrogen-pipelines