You know how you make the expertise in a large organisation?
You call for volunteers to run a pilot, move one team or a product to open source alternatives, learn what skills are needed in your tech people, what transition training is needed for staff
Have the pilot group select the desktop environment, change it if the choice generates too many tickets
Take that and roll it out more broadly, possibly aligned with new desktop hardware rollouts
Add to the good - you get to know that the US government couldn't lean on a single American company for access to your organisation's secrets