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.
As an armchair expert, I think it turned out badly because they had to develop cutting-edge technology (no trains with that top speed and support for gauge change existed before, and it also has other quirks, like being uncommonly wide to support five seats per row) but, at the same time, make it very cheap (the project started in the context of harsh austerity in the years after the financial crisis, with PIGS accused of overspending, etc.). They promised too much for the budget and ended up delivering a half-baked train. At the beginning, a year ago, it was a disaster (lots of incidents with trains stopping mid-way, etc.), now they seem to be ironing out the problems and things are getting better but they're still much more unreliable than other trains.
I hope at least the lessons learned help towards making a better model in the future.