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?
It's a slow and quite annoying process. For example, to reach my region, trains from Madrid have to change gauge because my region still has the old one. Apart from spending around 10 minutes doing this, this has caused a lot of problems because it essentially means there is a single model of 300 km/h train that can make it here (others don't support gauge change) and to top it, said model turned out to be highly unreliable. This created a lot of political tension because of course we wanted 300 km/h trains like other regions, but now we're stuck with these lemons and our regional politicians push for gauge change, but the national government doesn't want to do it yet as it affects freight trains.
I hope at some point we get the change done in the whole national network, although generally it moves at a glacial pace. It makes sense to have seamless connection with France and the rest of Europe, and to be able to use the same trains everyone else does.