On Tuesday, October 28, at 1.15pm, Tony Adams gave a press conference to announce that he was now manager of Portsmouth, a Premier League football club. He became at a stroke a member of the elite. But here's a much bigger date: Friday, August 16, 1996, at 5pm. That was when Adams had his last drink.

“Bill once told me that I was the best Guinness drinker in his pub. ‘Yes, what an accolade,' I thought. ‘What an achievement.' I had won medals and England caps all over the place but this made me really proud.” Adams truly had, as he claimed, “mastered Guinness”, and a 20-pint session was always within his compass.

The drunken scrapes, the crashed car, the jail sentence, well, we needn't go into that now. The pivotal point is that this was a life worth changing, and Adams changed. By an effort of will, he changed his life. Insofar as it is possible for any of us, he changed his personality. Adams changed himself.

These days, he goes to the theatre, plays the piano and reads poetry. He founded a charity for addicted athletes. He has not only rescued himself, he has continued, more or less unbroken, a career as an achiever. But all that he has done since 1996 has been the result of personal reinvention.

Times Archive, 1983: Tony Adams' Arsenal debut

Adams, a 17-year-old, got an early pass, nervously lost control, and the ball went to West, who hit a speculative long shot past Jennings

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If you don't like your life, you can have another. If you don't like yourself, you can be someone else. These are intoxicating thoughts: ideas that fly in the face of what we are taught. We are told that character is immutable, that our decisions are irrevocable. The idea that we can change everything and rewrite our destinies is thrilling.

Life-changers such as Adams can be found in sport more frequently than we might think, probably because sport has the knack of putting personalities into the Petri dish of stress before our laboratory eyes. You can find a second example of a would-be life-changer without turning more than a page or two of your newspaper.

Joey Barton, the Newcastle United midfield player, has become a new person, we hear. Barton is a footballer with a history of compulsive violence: a man no more able to resist the taste of violence than Adams was able to say no to that twentieth Guinness. The most notorious episode concerns the extinguishing of a cigar in the face of a colleague, but there are many other more humdrum tales.

As it was with Adams, these misdeeds led to a jail sentence. Since that last adventure, though, Barton has been singing another song. He has given up the booze - always helpful stuff for a man seeking violence - and now he is talking to anyone who will listen about the error of his ways.

He is making a manful effort to change and, whether his remorse is about the mess he has made of other people or the mess he has made of his own life, the fact remains that

he is actively trying to turn himself into the sort of person who leads a better life. There is a sense in which two personalities can live side by side in the same person. Roy Keane's book seems to confirm this: half of the person revealed in its pages is a hard-drinking man of uncontrollable temper driven by biting contempt, the other half is something alarmingly close to a spoiled intellectual.

No longer playing for Manchester United, Keane is now the manager of Sunderland. These days he radiates a personality of calm, wit, intelligence and - the last thing you would expect from the bulging-veined player - irony. He has found a way of combining the drive and ambition he had as player with a kind of inner peace, a truce - however temporary - with the warring elements within.

He too has given up drink. Addiction, its confrontation and conquest are the most obvious ways in which a person can change by sheer act of will. But there are other ways in which people can change that sport is adept at showcasing.

Andrew Flintoff was a fat waster who could get away with most things because of his bottomless talent. He could be as good as anybody simply by using his natural gifts, and for years, that was enough for him. But then his agent and his coach lost patience, and told him that he was a fool and worse. And Flintoff actually listened.

He became the pared-down, ferocious instrument of destruction that defeated Australia. By adding discipline and purpose to his life, he found that being as good as anybody was no longer enough. It was his will and drive in the summer of 2005 that won the Ashes for England and saw him recognised as the leading cricketer in the world.

Part of this change involved getting married and having children, events that force many people to reinvent themselves. Those who try to carry on without some fairly radical adjustments generally find themselves in some kind of trouble.

Andy Murray's sublime form over the past couple of months can be traced back to his decision a year ago to become the independent and autonomous boss of his own life. He sacked the expensively procured Brad Gilbert, set up his own entourage, and then had to justify things by winning.

Murray is but 21, and life-changing - in these circumstances it is called growing-up - tends to happen to people around that age, if it is going to happen at all. But Murray seized control of his destiny. That much is unquestionable.

I have seen athletes change their personalities before my eyes. I remember the evening when Andre Agassi changed from a fop to a tennis player of purpose; when he suddenly realised he could not only hit some flashy shots past Boris Becker, but that he could also beat him off the court. I remember when Martina Navratilova transformed herself from a podgy little thing to the most athletic performer that women's tennis had seen.

We have all seen life-changers in action. We all know people who have drunk too much and then stopped. We all know people who have lived sexually adventurous lives and then become model husbands and wives. We all know men - it is almost always men - who have made a terrible mess of parenthood first time round and, divorced, dote on their new children as if they had just invented the concept of family.

But that's why we change our lives - because we wish to unmake our mistakes. We never can. The people we damaged in the past can't be unhurt, but at least we can try and do it better the second time round. Oh, I am a great believer in the second chance: it's only when people start to need the fourth and the fifth chance that I start to lose patience.

All those who can take the chance to change their lives - their personalities - for the better are to be respected. Those in sport, who have done it all in public, show us the thrilling and dangerous truth: that we are not necessarily prisoners of our past, nor yet the helpless victims of our personalities.

* I have quoted from Tony Adams's autobiography, Addicted, superbly ghosted by Ian Ridley.