WHAT'S in a name, a logo, an image? Well, absolutely everything, which is why changing them costs so much. Get it right and re-inventing yourself can lead to new customers, new-found wealth and prosperity. Get it wrong and it can break you.

Times change and so do tastes. It happens for no explainable reason, but it does happen. And companies fail to change with it at their peril - just ask Marks & Spencer.

It happens at a conscious level - with customers noticing a change in livery, paper heads, lighting or decor. But more importantly, it happens at a sub-conscious level - where customers decide to like, trust, or admire a brand for reasons they can't really explain.

Never has the buying public been so image-conscious, so discerning and so difficult to please. Never has the market place been so full of rival brands, many offering the same thing, at the same price, with only the label to separate them. How important then is that label?

"The brand is everything," says senior lecturer in marketing at the University of Northumbria, Linda Danielis. "We all have expectations with particular brands. If it's a company name, in the public domain, you will find they will put a lot of money into it. The new image is reinforced and new buyers will forget about the old name."

The ultimate aim is to become ingrained in the human mind. Brands which become generic have the system cracked. If you need a cheap disposable pen, chances are you will ask for a Biro. If you require liquid paper, you are more likely to ask for Tippex than any other make and if it's sticky tape, then it has to be Sellotape. And there's very little the competition can do about it.

Change any household name for corporate reasons and you are likely to upset people who tend, on the whole, to be pretty conservative.

"Marathon became Snickers and I'm very surprised it hasn't been rejected," says Mrs Danielis. "I was always brought up with the idea that it was a good name. Now I can't help thinking knickers. It's really very, very important to get the name right. In people's perceptions, the name is the brand and that is the thing which becomes marketable."

Changing the name to fool the public wins no friends, she claims. If the product isn't very good, the consumer will remember it, whatever it's called.

People also remember notorious names. Windscale tried in vain to wipe the slate clean with a name-change to Sellafield, hoping the public would forget about previous radioactive leaks form the nuclear reprocessing plant. It failed.

More successfully, the Burton group changes its image subtly every three years to appear more modern and fresh without altering its look radically.

"Heinz spends a fortune on this, keeping its design as close to what it was before, but making it significantly different to make sure it looks up to date," Mrs Danielis adds.

"But M&S are suffering at the moment. They seem to have lost direction and don't seem to know what the customer wants. I should be ideal for their market but find myself coming out of the shop empty-handed."

Care also has to be taken on the international market as translations can be disastrous in the wrong country. Buy Hyundai's little supermini, the Atoz, outside Britain and it is badged the Atos. Here the name was dropped once the concept "don't give a toss" was explained to the Korean marketing department. Get it wrong in the car world and you are likely to become the butt of countless jokes, as well as seeing your profits suffer in the sales war.

British Airways got it badly wrong when it spent a fortune on arty designs for the tailfins of its planes. The aim was to appeal to non-British customers. But dropping the union flag actually alienated loyal passengers, the British, causing the carrier to think again.

Levis conjures up an image of out-doorsy denim, hard-wearing casual-wear. But in branding a range of designer tailored suits, Levi flopped in spectacular fashion. "What it needed was an Italian sounding name," Mrs Danielis says. "But at least they held their hands up to the mistake."

Outside the consumer market place things are slightly different, with one corporate company needing to appeal to another for business on the world stage. This is what has prompted the Post Office's move.

Becoming Consignia later this year is designed to show the changing nature of the organisation. In this country, Royal Mail and Parcelforce brands will remain and there will be no change to the network of Post Office branches. Shop front and postal workers' uniforms will also be unaffected by the change.

But from March, it is hoped the new name will give the company a modern and meaningful face, when the Post Office is given greater control over commercial decisions and more freedom to borrow and invest in services.

It will be seen principally by corporate customers in the financial services, telecommunications, home shopping, utilities and advertising and marketing sectors, which account for nearly a third of its annual £7.5bn turnover.

A lot of thought, effort and research went into the new name, according to a spokesman. Management and staff were polled, along with customers, suppliers and the unions. The research was carried out at home and abroad.

The spokesman says: "The research audience was given a description of the company but we didn't tell them who it was, to see if the name was suitable. They said it sounded prestigious and modern." London marketing consultants Dragon Brands designed the logo.

There were also concerns that the name, the Post Office, was generic, couldn't be used abroad, nor be legally protected. It failed to differentiate the organisation from any other postal administration.

The Post Office's new name will cost about £2m to implement, which sounds a small fortune to the lay man, but has been defended as reasonable by the management, given the riches to be found in the global market today and the rewards at stake.

If the name fails to succeed and is consigned to the rubbish heap, the Post Office may find itself being asked why it didn't spend a bit more on the new name. But that's marketing for you.