DirectX 12 shouldn't have any impact on existing games aside from perhaps MMOs that are constantly in development and are adding new features all the time. A game has to be written to take advantage of an API, so current and old titles that support DirectX 11 will only support DirectX 11 and won't benefit from DirectX 12.
In any case, DirectX 12 games are still a few years off, as games take time to develop, and many developers won't be rushing to add features late in development that only a small portion of the userbase with the latest hardware and OS can use. For example, DirectX 11 was introduced in late 2009, only a couple of games came out in 2010 that supported DirectX 11, and we didn't see a significant number of titles support DirectX 11 until 2011 (even then a lot of those titles didn't add DirectX 11 support until a post release patch). As such, DirectX 12 will be introduced later this year with Windows 10, we may see a couple of games support it in 2016, but it isn't going to start to see anything close to wide adoption until 2017 at the earliest.