Although World of Tanks has been updated many times since its introduction, most notably gaining DirectX 11 support, the game's 3D engine is due for an overhaul. Of course, Wargaming.net also has an obligation to not abandon players with lower-end systems, since they're a huge market targeted by the company's games.
With the new Core engine, Wargaming.net appears to have realized its goal. The company says the game's minimum required settings remain unchanged, and graphics quality at the lowest preset appears comparable to today's version of World of Tanks (that is to say it's mediocre by modern standards).
But if you own a newer graphics card, Core should effectively utilize its functionality to serve up better-looking visuals than anything the game offered before. When can you expect the switch to happen? Wargaming.net says, "We will announce the exact date later. However, the release of World of Tanks enCore shows the switch is going to happen soon."