"Worst" case scenario, they would all be subject to the same visa rules that non-EU use.
So that means a simple process for the already employed. A few months confusion for business owners, an easy application for those who have been there for many years, and some of the more recent migrants may have to return.
The majority would stay, and the majority of EU nationals living over here would stay too.
Its not about booting anyone out, its about controlling the number entering.
The already employed wont automatically get a visa, they wouldn't have a right to a visa like that. The country of residence would have to set up some special agreement for British citizens already living or working there. but why would they, If Britain restricts that counties nationals' free-movement to the UK then why would they not want to have an equal treatment?
And that completely ignores all the Brits that have retired abroad, what kind of visa do they get?