If you have to go from a smaller gauge to a bigger one you most likely have to also expand corridors, buy new land, fix bridges and tunnels etc...
While a downshift if usually much esier since a smaller gauge simply fits inside the larger one so all bridges and tunnels are wide enough by definition.
That's mostly an issue with loading gauge, not track gauge. The train is usually significantly wider than the tracks, so width-wise the space should already be available.
