You can create a time paradox with the new achievement/ trophy.
Eurogamer's Ian Higton plays through the mission that has you kill Zakhaev in the video above.
In a flashback sequence in Modern Warfare 3, the protagonist Yuri is in a jeep behind Zakhaev along with Vladimir Makarov, and watches the shooting play out.
In the original Modern Warfare, Yuri and Makarov aren't in the jeep, but in the remaster, you can see them sitting there as clear as day, and this is where the time paradox comes in.
After shooting your target Zakhaev, you can also shoot and kill Vladimir Makarov, thus creating a time paradox that disrupts the events of Modern Warfare 2 and 3, in which Makarov plays a part.
This will net you the aptly named trophy/ achievement Time Paradox.