It doesnt really matter if the UI is the same or almost the same. We are talking about rolling out an entire new suite of tools to every government worker in a given country.
It would take years to plan the migration, then start deploying it in stages to the less critical services of said government while you work out all the kinks.
While that is happening, you most likely will need to support both systems for a while so that you have a fallback if things go wrong with the new system.
Then you will need to train all these employees properly so that they can potentially troubleshoot some of the issues they encounter and give them enough time to become as productive as they were before the new system rolled out.
> But at least in Germany, I've seen Windows being written into agreements between state governments and trade unions representing clerks and employees. Good luck changing those without a negotiation running 3 years.
And there is that too. I am sure its the case in many countries.
Finally this migration if successful would not bear any fruits to the person who initiates it at best because by the time the roll out is complete this person will have moved on to another position, be elected somewhere else or retired.
But if it goes badly, you can be sure that this will be used by the opposition political parties to attack the governing party and accuse them of bringing the entire country's services to a stand still.