So should buyers wait for the second letter? Will new and used cars become cheaper, or more expensive? Which will be the busier month: March or September? Could cars depreciate in value much faster? This revamped twice-yearly registration system seems to raise a lot of important questions.
First of all, why the change? The Society of Motor Manufacturers and Traders' (SMMT) official line is that the old 1 August registration system simply did not work.
"One change of letter a year puts enormous pressure on manufacturers to produce a huge slice of the year's output for one month," a spokesman says. "Around a quarter of all annual car sales are in August. So 500,000 new-car sales means 500,000 used-car trade-ins, which depresses car values."
It also meant that buyers were short-changed. They got less money for their part-exchange and there were doubts over some dealers' ability properly to prepare so many new cars at once. Not only that, what was sold was almost obsolete stock, because manufacturers traditionally launch revised and better-equipped models in September and October to coincide with the Motor Show.
In theory then, the twin peaks of March and September should be good news.
Manufacturers have been doing all they can to encourage buyers into S- registered cars and there have been some amazing deals with free insurance, servicing and finance packages. Around the country, clutches of delivery- mileage vehicles, registered to boost sales figures, are now up for grabs.
World of Cars, in Suffolk, has a batch of S-plated Fords, with savings of more than pounds 4,000 on 1999 Mondeos and Fiestas. Even so, car sales in January were down at 181,842 against 232,055 in January 1998, although that was a record year. According to Paul Everitt, the SMMT's head of policy and economics: "The January registrations reflect the current economic climate and anticipate the arrival of the T plate in March."
"This is a transition period at the moment," says Paul Jarvis, managing editor of Glass's Guide, the trade's price-guide bible. "Consumers are only just becoming aware of the changes. However, we see March becoming the dominant month. The conditions are ideal for this, as Christmas debts have been settled and summer is coming.
"As for September, there will still be a desire for the latest registration plate and we will be listing three price blips in our publication for January, March and September."
CAP, Glass's price-guide rivals, sees things differently. Ramesh Notra, economic analyst at CAP, says: "We are going to see very significant changes. For example, anyone who continues working on the assumption that spring time will be uniformly better than the previous year-end is in for a rude awakening."
CAP expects the September plate change to have more impact than March. It also believes that seasonal factors will put more downward pressure on used car prices in 1999, especially towards the end of the year. CAP also warns that two plate changes a year may lead to faster depreciation as the latest letter premium is lost twice as quickly.
Consequently, buyers won't be so tempted to consider a change for the sake of a rapidly changing letter, which may result in fewer new car sales. As a result, CAP predicts there is boom time ahead for cherished number plates: "Many private buyers who do not want their new cars going out of date twice as quickly will be tempted to buy these plates."
Tony Hill, who runs Elite Registrations - one of the country's largest cherished-number dealers - says: "I haven't noticed a surge in demand because of the new system."
Obviously, everyone is on a steep learning curve. The dip in January new-car sales actually meant fewer part-exchanges and an unexpected rise in used-car values, as dealers could not find enough stock.
We will certainly know all the effects by the time the alphabetical system comes to end, with Y-registered cars in March 2001. After that, a whole new number-and-regional-identifier plate will bring with it a whole new set of problems, questions and implications.