How is a change like this going to be implemented? E.g. are they going to mainly update some tracks everywhere (and have two systems running in parallel), or all tracks in selected areas (and have passengers change), or something else?
Was there a comparable large scale rail infrastructure change in some other country?
There were a number of gauge changes, but they were usually quite early on, when the infra was less critical and you could get away with closing lines for months. I'm not sure that there's a real 20th century example, beyond standard gauge high speed alongside non-standard normal-speed (for instance see Spain, and likely soon Ireland).
It was also a time when railways used wooden sleepers, so you could simply drill new holes at the new track gauge for moving the rail fasteners, thereby minimising the work required for changing the gauge, at least on the plain line, switches and crossings excepted.
Plus it was a time when a lot more manpower for that kind of massive manual work was available, plus railways were the dominant transport mode and could actually commandeer that kind of manpower.