Drivers of any non-compliant vehicle entering the zone after 4 February 2008 were going to get a Penalty Charge Notice. Now, however, they're going to receive receive a warning letter giving the vehicle keeper 28 days to make the vehicle compliant instead.
Why? The reason is a Freight Transport Association campaign that said that it was very hard to accomplish such a target because of difficulties that freight operators have for replacing old vehicles. The Eastern Europe demand for commercial vehicles resulted in a shortage for Western European markets.
The measure is also flexible: if the operators can't really find a replacement vehicle and have made reasonable efforts to comply but have encountered problems, they will be offered help by Transport for London to fix the situation.