The A2 has had a vast array of improvements since Classification. It is believed that the A2 had the earliest renumbering due to a building of a new road, in this case the First Dartford Bypass. There have been many more that that though since this is the road to the channel ports.
The original draft route of the A2 ran from London to Rochester via Welling, Crayford, Dartford and Gravesend (now mostly the A207 and A226), which had been the historic coaching route for centuries, not least that used by the pilgrims mentioned in Chaucer's "Canterbury Tales". It mostly followed the Roman Watling Street aside the section from Dartford to Rochester via Gravesend.
Between the draft and the final allocation of the route on 1st April, 1923, construction had begun on the western section of the original Rochester Way as far as Welling, and an upgrade of Watling Street to bypass Gravesend. Consequently, the final route of the A2 ran on these new roads, with the sections of the old route becoming the A207 and A226 respectively.
In 1924, the First Dartford Bypass opened and thus the A2 was rerouted from the congested centre of Dartford. Although road numbers had already been pre-allocated to road projects under construction, the renumbering of the A2 was one of the first renumbering specifically due to a building of a bypass. Another re-routing occurred in 1927 with the extension of Rochester Way from Welling to Dartford.
These early improvements were all wide single carriageway roads, as was standard practice for new routes in the 1920s.
Post War (1945-1977)
After the Second World War, very little changed at first. It was still mostly S2 however as soon as the Preston Bypass opened in 1958, big upgrades began to happen. In 1963, the M2 opened and the Medway Towns were Bypassed. This changed Brenley Corner from a major T-junction to a GSJ Roundabout. The Western end is where the D2M M2 became the S3 A2. It was like this until 1968 when it became D3M, a standard far higher than the motorway. This an oddity that lasted until the late 1990's with upgrade of the M2. The next year, work began on the Second Dartford Bypass and the Dualing of the eastern part of Rochester Way, Parts of witch finished earlier. In 1972, the works to build the Second Dartford Bypass and the upgrade of Rochester way were finished.
At the country end, Lydden and Temple Ewell were bypassed in the 1970s. In 1977, work finished on the Whitfield Bypass and Jubilee Way, road that bypasses the eastern parts of Dover.
Modern Upgrades (1988-present)
After 1977, things were a little quieter on the upgrade front. The Canterbury Bypass opening in the meantime. The next major upgrade of the road was the new build Rochester Way Relief road. Bypassing the now urbanised Rochester Way, the Rochester Way Relief road was a D2, running from Falconwood to Kidbrooke. In the 1990's, the main upgrades were off the road as the Bean Interchange and the A256 Junctions were upgraded to modern standards. It was to be the 00's when the big upgrades happened. At Park Pale Interchange, the old layout was replaced by a trumpet-fork junction that finally killed off the D3M A2 to D2M M2 layout. In 2003, Phase one of the Bean to Cobham improvement was complete. It turned the D3 A2 into D4M. Though big, it wasn't as big as phase 2 where the A2 was rebuilt onto an all new lineament. Taking three years to build, it made the A2 D4M from Cobbham all the way to Bean. It also involved works at the Darenth Interchange to make some of the slip roads more free-flowing. When the works finished in 2009, the western end of the A2 had become motorway in all but name.