Yes, those kind of track relaying machines exist, and they could most likely be adapted to work for the gauge change (one end of the machine would have to be set up for broad gauge and the other end for standard gauge), but they still manage only a few kilometres of track per day, and they're comparatively rare and expensive – basically there are only enough of them around for the amount of track relaying required during routine maintenance.

So repeats of the famous 19th century gauge change by converting large swathes of the network in just a few days (thousands of miles of track as in the US in 1886, or even just the 177 miles west of Exeter in the UK in 1892) remain rather unlikely.