IF it ain't broke, don't fix it. Normally good advice, but not for automakers. In the hotly competitive new-car market, tastes can change overnight. Even the most popular cars don't stay hot for long. The Ford Taurus is one of those rare exceptions. Since its introduction in 1986, sales have kept going and going, making Taurus the nation's best-selling passenger car for three years in a row. So, it's not surprising that the last time Ford updated the Taurus, it took a trained technician to notice the changes. But there's no way to confuse old from new with the 1996 Taurus just now reaching dealer showrooms. Inside and out, the '96 is a markedly different and significantly improved automobile. But the real question is: Will it still have the magic it takes to stay on top of the charts? A decade ago, the Taurus was a styling trendsetter, introducing America to the fuel-efficient ''aero-look.'' It was the ''jellybean car'' to some, the ''turtle on wheels'' to others. Today, however, it looks, well, almost quaint. Competitors like the Chrysler Concorde have pushed the design envelope. But the king-of-the-hill isn't giving up without a fight. The new Taurus has been radically reskinned. The active word is oval, from the elliptical headlights flowing gracefully into the hood, to the curvaceous rear window. Though striking, there are some trade-offs: The rake of the roof means limited rear-seat headroom, and the trunk is smaller than before. Ironically, the whole car looks tinier, but it's an optical illusion. The interior actually is 17 percent larger than previous models, and the body is five inches longer and nearly two inches wider. The biggest surprise is the '96 Taurus wagon. Anyone who thinks of a wagon as mundane better take another look. This cargo-carrier boasts an even, more radical redesign than the sedan. It is a risky choice, which could alienate repeat buyers. But as Jacques Nasser, head of product development, puts it, ''We figure that if you're comfortable with a design right away, we didn't go far enough.'' The ovoid look of both the Taurus sedan and wagon is maintained inside and out. One of the most striking features of the new Taurus's interior is its Integrated Control Panel. This egg-shaped ICP brings within easy reach both the climate-control system and the stereo. But it's more than just a new look. The ICP is actually a stylish remote control panel. The actual hardware has been moved to other parts of the car, freeing up the costly ''real estate'' behind the dashboard for other applications. It also makes it near-impossible for thieves to get your stereo. Cornucopia of features The car comes with a cornucopia of features and technical improvements. Start with a much-improved powertrain. The base sedan comes with Ford's long-running ''Vulcan'' V-6, which has been a rough but reliable mainstay. For the '96, the engine becomes, well, tame. It's quieter, smoother, even a bit faster. If they can afford the step-up, most buyers are likely to opt for the high-tech ''Duratec'' V-6. It's got significantly more power - and you don't have to worry about a tune-up for 100,000 miles. The new Quadralink suspension, which shares features with Ford's top-of-the-line Lincoln Continental, provides more accurate and predictable handling and a stiffer body. Lofty Goals Ford had some lofty goals in mind for the '96 Taurus, aiming to beat the Toyota Camry in terms of personal comfort. Camry is the midsized-car benchmark for automakers. Consider the triple seals used on the Taurus doors. They're one reason the '96 is significantly quieter than the old Taurus - though not quite up to the pin-drop silence of the Camry. The other challenge is quality. From the start of production, Ford promises Taurus will deliver 20 percent better quality than benchmark Camry. That's quite a pledge, considering most cars go through a quality shake-out in the first few months of production. Even Toyota's new Avalon slipped into the sub-par category according to J.D. Power & Associates, during its first year on the market. Will Ford be able to do any better? The numbers should start rolling in by next spring. For many buyers, the bottom line is, well, the bottom line. New-car prices have soared to a record average of $20,000, pricing many potential customers out of the market. In recent years, Ford hasn't been shy about moving up-market, and the new model jumps to $19,150, for the base Taurus GL sedan. Ford defends the increase by pointing to all the equipment added to the 1996. Perhaps, but that 5.6 percent bite may be a bit much for some potential customers. After the initial shock, the Taurus should feel surprisingly familiar to anyone who's driven the car before. Just a little bit better.