IMO, the reality is that these plants will never convert to anything other than natural gas and will eventually be shut down, still running on natural gas.
Hydrogen is not a fuel, it's an energy storage mechanism, and not a very good one at that. If you're going to store energy there's much more efficient and/or cheaper ways of doing it.
Natural gas is still a good fuel but I don't think anyone in the US at least is under any illusion that these will ever run on anything other than natural gas.