We are talking about Government Offices here. The UX difference between Word and LibreOffice is noticeable.
People need retraining, no matter how facsimile both are. Its matter of productivity.
Yes, the UI is different, but the concepts you're dealing with are still the same and if you know the concepts then a new UI is just a small and temporary inconvenience, rather then a barrier.
The biggest problem will actually be incompatibilities, which happen for example with advanced usage of Excel documents, but you know, in such cases it's better to switch to portable formats that work across multiple products (i.e. ODF) and it's better to do it sooner rather than later, because as a freaking government you should keep your documents accessible for everybody. And if you switch to portable formats and a set of features supported in both Microsoft Office and LibreOffice, you don't even have to move everybody to LibreOffice at once. So on the whole I feel that this is a red herring.
Why do government workers always need formal retraining? Every job i've ever worked involved constant intake of new skills without dropping tools and attending a class. For a new OS in an office environ that generally boils down to "Here is your new OS. Play around with the colours if you want, but that memo is still due Thursday."