Finland announces migration of its rail network to international gauge

Doing this is a really nice opportunity for Finland. The should bundle it with many other upgrade. Doing general maintenance. Upgrade all the signaling (ETCS L2). Overhead electrify everything. Do minor speed upgrades. And so on.

If you are not doing all of this at once, this likely isn't worth it.

I’d imagine that would make planning and implementation more complex

Sure but if you already spend work teams along every railroad in the country, it would be a waste not do everything else.

If they build new lines next to existing ones, they need to touch all that stuff anyway. No point in replicating old systems.

The signalling you are describing is more complex and possibly more expensive than switching rail. Source: I work in rail traffic engineering.

The idea is simple. Ensuring everything is smooth and safe = cost multiplier.

They will have to do it anyway in the same timeframe.