You need to upgrade those to be able to use hydrogen, but you don't lose a lot more efficiency if you upgrade the hydrogen to methane afaik.

Depends on how decentralized you can inject hydrogen from overflowing renewables, then. If you could do so at the city's connections to the main long-distance network, it might not be economical to have the infrastructure to create methane from it, and, on top, the hydrogen fraction would be CO2-free even if burnt in a normal boiler/stove. I wonder what upgrades would be needed, though? Are they not already using one of the steel alloys that is resistant to hydrogen embrittlement?