Here in the US, H2 is certainly cannot be used in place of methane for heating. The current underground pipelines leak too much as it is - and that's a much larger molecule than hydrogen. On top of that, the pipes are designed to resist degradation from methane, not more-reactive hydrogen.

The best possible reuse of the pipelines might be for geothermal.

Hydrogen might eventually be usable as a vehicular fuel. In that case, the entire system would be redesigned specifically for it. Agreed that right now, EVs look to be the better solution, but there could be some limits to growth there (lithium availability, grid's general 'creakiness).