Hydrogen doesn't just require "modifications to the gas mains", it requires a complete reconstruction, and it'd probably be a really dumb idea. Hydrogen embrittles metals. You put it in any sort of regular pipe, and your system will start springing leaks everywhere from distribution to end-user consumption. It also leaks through almost everything, but especially things not specifically designed for it. But it gets worse, because after it leaks it tends to pool in explosive mixtures under overhangs. Also, if you have multiple pipes running parallel, and there's hydrogen in the lower one but not in the upper one, part of the hydrogen leaking out of the lower pipe ends up in the upper pipe, where it can follow it to its destination and pools there. Beyond that, H2 has combustible fuel air mixtures way, way wider than of methane, 4-75% in air. And unlike methane, it can readily undergo deflagration-to-detonation transitions under STP conditions. NASA safety guidelines require any facility handling more than a dozen or so kilograms of hydrogen to have a roof designed to be blown away in an explosion. And hydrogen ignites with a tenth the ignition energy of methane. We're used to fuels that require a visible, audible spark to ignite, but hydrogen ignites with the sort of tiny static or electrics discharges that you don't even see in everyday life; ordinary electronics are not designed to be safe in an environment where a combustible hydrogen mix might leak into.
Beyond that, producing hydrogen then burning it is a ridiculously wasteful approach. Even using it in a SOFC after producing it is still ridiculously wasteful. And it's also a very expensive process. Producing methane from atmospheric CO2, however, is so bad it makes even hydrogen look efficient by comparison.
Obviously, the efficient way to store electricity is batteries. Given DC and not too fast of a charge rate, li-ions, for example, can be over 99% efficient. But obviously the price for storage would be way too high. There's various cheaper techs on the market, including some forms of flow storage, with radically cheaper ones in development, and there's talk of using used EV batteries for grid storage; we'll have to wait and see how that plays out. Also far cheaper and more efficient (~75% net) than hydrogen production is pumped hydro, with or without a river present. Compressed air storage is relatively cheap, but inefficient (~10-30%); however there's some lab-scale attempts at isothermal storage which might get that signficantly higher.
Sometimes you see claims on hydrogen or compressed air production that are higher efficiency, but that's just PR flak; they get those numbers by assuming you make use of the waste heat for some other industry that would otherwise have to burning something to produce said head. But you can say that about every system on earth, because everything has waste heat. The number that matters is how efficiently you can store your electricity.
Who's talking about replacing natural gas wit hydrogen? That is what you mean isn't it? I will admit that I'm no chemist nor an expert in the effects of hydrogen on pipe material. All I know is what I have read in the energy industry journal I subscribe to. The idea as far as I understand it is to supplement compressed natural gas (CNG) with hydrogen. That's safe with minor modifications up to a mixture of about 20%. I remember reading somewhere that this has been field tested in Holland where they found that mixtures of up to 20% hydrogen and 80% CNG (by volume) could be treated almost identically to pure CNG. Excess energy from wind and solar plants goes to waste 100%. It does not matter that the process of using that energy to create hydrogen using from that SNG is wasteful, at least the excess energy is not going completely to waste the only issue is whether or not it can be done economically. Making P2G economical is a big issue and It won't be easy but then if it was easy everybody would be doing it. And I'm not quite sure where you are going with that Battery idea, the thought here is to store thousands of gigawatt hours of energy (the current German Gas storage capacity is 200,000 GWh) and that would require one massive battery.
