Finland announces migration of its rail network to international gauge

> will cost billions of euros, affect more than 9,200 km of track, and take decades

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?

Currently the leading plan is to build another narrower track alongside the existing ones (so the old trains can keep operating), but it is still in the planning phase. [1] I am not convinced this project is ever going to pay for itself. I feel like you could move cargo from one train to another somewhere near the border for quite a long time with the money it is going to take to convert the entire rail network. Finland is only connected to Sweden and Norway by land in the North so it's not really going to connect the Finnish rail network to Europe either (unless the Helsinki-Tallinn tunnel [2] gets built, but it does not seem likely at this time).

[1] https://yle.fi/a/74-20161793

[2] https://en.wikipedia.org/wiki/Helsinki%E2%80%93Tallinn_Tunne...