Article mentions that Reboot was recorded right after Changmin’s discharge (8/18) at Japan’s Press Conference (8/21):

According to it, it seems Tohoshinki’s Reboot was recorded after the 8/21 press conference in Japan. At the sound of their voices, it was said one couldn’t feel (the effect of their) absence and the recording process had gone smoothly.

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Credits:

Daily Gossip via @iruka0206,
Translated by @snxy,
Posted and Shared by TVXQ! Express