Hmm... This really should be easier to do... even if Upwork doesn't want freelancers to be able to change the country of residence indicated on their profile change on their own... At least make it clear how to ask for help doing so.
Provide a standard button or something that can be clicked to kick off a change-country-of-residence process.
Maybe providing that would make it even more likely that freelancers' profile information will be accurate.
Especially if said easy-to-use button would trigger a process that requires authentication of he country of residence through an Upwork employee.