When talking about using existing pipe lines I have my doubts...

When talking about using existing pipe lines I have my doubts whether this is feasable due to the way Hydrogen affects steel.

Yes that is the main issue which is limiting the use of the DBNG pipline to about 10% v/v at present. Certain grades of steel are more susceptible, as are certain welding processes. A higher partial pressure of hydrogen (operating pressure x volume fraction) is also a big factor. To a degree they can reassess and retrospectively qualify the pipeline materials and welds for some h2 service, but inherently they can't change the materials which were already installed.

Since H2 has a very low molar mass compared with natural gas (2 vs 19), 10% v/v isn't actually that much when you look at the energy content (the thing that matters), even after correcting for the higher calorific value. 10% v/v is just 2.7% by energy content if you do the sums.

Any new piping can be designed and built for H2 service without too much hassle, just a bit more cost involved for the higher spec materials.