I think we would need to change the immigration rules, Just5, though as you say that can be simple enough
There is also the fact that the High Court has repeatedly made governments (of various persuasions) abide by principles of fairness in not introducing immigration rules that would have a retrospective impact. So I would be pretty bullish about the ability of the vast majority of EU citizens to remain in the UK (though accept that's easier to say when it's not directly impacting me)
I do get annoyed at pp throwing around terms like fascism and making out that the UK treats immigrants worse than anywhere else in the world.
Firstly, every country has the right to decide who lives within its borders - in and of itself, it is not fascist to determine this. (It might be racist or objectionable in other ways, of course.)
Secondly, we are so much more welcoming, tolerant and accommodating than so many other countries: we don't limit property ownership to British Citizens; by and large we don't exclude non-Brit Cits from working in certain jobs or professions; we allow dual citizenship; we are generous in the amount of time you can be outside the country but still clock up time towards permanent status; we allow access to education, public services, healthcare etc etc etc.
personally I'm proud of all this, despite the continuous rhetoric about being tough on immigration, and the constant tightening of the immigration rules and policies. But please don't make out that the UK is the worst place in the world to be a foreigner - it really isn't