Conversion to a narrower gauge should be a fairly straightforward process, unless concrete sleepers are in use. New axles or outright replacement of trucks shouldn't take multiple years of effort.
We did such things in the US in month long long ago.
You ignore the scale of the project, still present an estimate and compare the effort to something irrelevant, from a very different time and place. Are you my PM?
We did such things in the US in month long long ago.