May I inject a note of maths here.
The virus may mutate. It also might not. Chances are it will, most viruses do, so let's say there's a 90% chance it will mutate (for the sake of argument). Let's also say that at the point it mutates 30% of the population have already had version one and developed immunity to it (still with me - not full immunity, they might get a very mild version - quite commonly - but not the full blown shebang so let's not particularly worry about those folk).
Okay so 70% of the population are completely unprotected from this virus. If it mutates it will change in it's virulence or it's effects. Let's say it changes virulence - so it's more contagious - it's done that before when it started to transfer between humans so that's quite likely to happen. Right now it's not actually that virulent, if you are in a room for under an hour or a few feet away and/or the surface the virus lands on is hard it can only survive a few hours, and if it lands on your sleeve, well that little fecker has only 20 odd minutes to get sucked up your snozz before it dies.
It could also change effects. And let's say it does. Let's say that instead of overwhelming the little hairy filaments in your bronchial passages (which it does right not I believe to allow it to flourish) it say attacks the pericardial sac (the outside bit of your heart). Now that's more serious I agree, but it'll attack a different part of the population - ie old folks, very young folk and people with dodgy hearts.
Now remember to achieve this it's got to come into contact with something that does this and genetically bond with it in order to mutate and it's got to come into contact with another host in order to transfer (providing of course it's stable enough to do this - and virulent enough to survive the transmission). That's three or four conditions depending on how you look at it that it's got to go through before it has successfully changed. But yes it may well do so.
Conversely, it could change to give you a nasty case of farting. Yes, that would be bad. But you aren't going to die. Although it would probably help with encouraging quarantine.
What I don't get is why we can't commit to a voluntary quarantine? Close the fricking airports and take anyone who needs care into hospital and just bloody well stay at home for two weeks. We're an island (and yes I know the 'well as soon as you opened the airports again argument too - everyone else would have to do it too).
Viruses can't live without hosts - think of all the nasty bugs we could kill off if we did this - it'd be marvelous. Yes I know the 'country would grind to a halt' but we manage to do that every time it snows or it's christmas - why not?
Everyone stay home. I'd vote for that - I can bear my family for two weeks close contact! (Just)