Advantages to Switching over to FOSS
The biggest advantage I see to open source for government work: the formats must, by definition, also be open. That means that their data can never be held hostage by any company.
If they're using Office, they're more or less forced to do so in a way that Microsoft finds acceptable. If they don't like the old deal any more, then when it's time for a new contract, the government can find itself over a barrel, with a very serious problem.
And citizens that want to interact with the government may be forced into relationships with Microsoft. It strikes me that requiring a relationship with a corporation in order to participate in your own governance is rather frightening.
To my mind, governments everywhere should be using open source exclusively, even when it costs more, and even when it's less convenient. If they're smart, they can cooperate with each other, to help save costs and drive the software to where they collectively need it to be.
It seems to me that record keeping should be a central competence of government, and writing the software to do it would also fall under that core goal. Under no circumstances should public records be behind a corporation's paywall.