Last I knew the Boring company wasn't making full size tunnels, but weird undersized tunnels that just barely fit a Tesla and have zero room for emergencies.
That means in order to make the tunnel wider you'd have to destroy that concrete layer over the whole length and add a new one. I am not an expert, but my guess would be that drilling a new one and using the smaller one as a maintenance tunnel might be more reasonable and cheaper.
Those tunnel boring machines that dig and put the concrete etc up are super cool though