Don't panic..... OK, panic a little bit.

Imagine you returned home from work today, to find everything in your house had changed.

Your sofa replaced by hard-backed, Japanese-designed chairs. Your squat, furred-up kettle gone, a tall, gleaming, stainless steel model there instead.

You still have somewhere to sit, and can still have a cup of tea while you ponder this sudden, unwanted change.

That’s what’s going to happen to your phone today. Assuming, that is, you have an iPhone.

While all the publicity in the last couple of weeks has been about brightly coloured iPhones, and fingerprint-scanning iPhones, the real revolution is a total redesign of the software that sits on those phones, and most of the tens of millions more already sold around the world.

Today’s the day iOS7 arrives, and it’s a far bigger gamble than coating the phone in lurid-coloured plastic.

One of the biggest selling points of the iPhone has been that they’re easy to use. OK, maybe your grandmother would need a little help. But when you consider it’s a spectacularly complicated mini-computer that just happens to also make phone calls, it’s surprisingly easy to navigate.

And that’s largely down to the operating system - the OS. Something most of us haven’t really thought about, or ever really had to.

For most people, iPhones just work. You pretty quickly establish the combination of tapping and swiping needed to get it to do what you want. And the basic method of working an iPhone hasn’t changed since the first model appeared six years ago.

Now, suddenly comes huge change, for huge numbers of people, all at once. It will look, and feel, very different.

Expect to see anguished expressions on trains, as commuters struggle to figure out how to reply to an e-mail or text message. Even the occasional smashed phone can’t be ruled out, should the frustration grow too great (maybe that’s part of the plan in releasing the new OS two days before the new phones).

So why do it? Why put so many people through the anguish of unwanted change?

Blame the geeks.

For years, Apple was beloved both of ordinary consumers, and nerds. Both liked the stylish look of Apple’s products, but the nerds also drew pride from belonging to a techno-elite.

Apple accounts for only a tiny proportion of worldwide computer sales - Mac owners would knowingly nod to acknowledge each other in coffee shops.

Suddenly that’s changed, because of the iPhone. Suddenly, that once-so-rare logo is everywhere. And in chasing the mass market, Apple’s had to keep things simple.

The geeks don’t like simple, so they’ve drifted away. Chiefly to Android, Google’s rival OS. It’s on many more phones, some of them cheap and nasty but others elegantly designed , and often capable of things iPhones cannot do.

Apple's bosses, stung by claims they've lost their innovative edge since Steve Jobs' death, have responded by ordering lavish new clothing for their phone -- though it's an outfit that looks like it's been made from somebody else's clothes.

Most iPhone users don’t want to fiddle around with the settings. They just want a phone that works, that lets them take pictures, slice open fruit with pretend swords, and brag about it on facebook.

Suddenly they have to worry about settings, about how to switch on bluetooth, or find a photo.

This is Apple’s big gamble. Not minor redesigns of the iPhone’s exterior - a radical shake-up of what appears on its screen, and how it gets there.

Prepare to spend the weekend scratching your head, and try to resist the urge to throw your phone against a nearby wall...