Denmark is not migrating off Windows/Office for any cost or technical reasons, they are doing it because they fear being over-reliant on technology solutions controlled by entities outside of the EU:
Because they're concerned about who controls European data, who sets the rules, and who can potentially cut off access to essential services in times of geopolitical tension.
But, of course, Linux advocates will claim this proves Windows is inferior, and Denmark will save countless millions of dollars/year... but neither is likely true.
The Windows/office ecosystem offers lots of capabilities that just aren't there in the free offerings out there, though most can be recreated with effort and some expense.
I don't think Denmark will save money, but I don't think costs will increase either, it will most likely be about a wash... Money that was spent on MS software licenses will be converted into software developers and training, and I suspect there will be a not insignificant number of mission critical applications that will not be easy to port to Linux.