https://www.whitehouse.gov/blog/2016/10/31/digital-transition-how-presidential-transition-works-social-media-age

In the excerpt below, NARA is the National Archives and Records Administration.

"On Twitter, for example, the handle @POTUS will be made available to the 45th President of the United States on January 20, 2017. The account will retain its more than 11 million followers, but start with no tweets on the timeline. @POTUS44, a newly created handle maintained by NARA, will contain all of President Obama’s tweets and will be accessible to the public on Twitter as an archive of President Obama’s use of the account. In addition, President Obama’s tweets will also be archived at NARA, where they will be preserved and accessible in the same manner as all other Presidential records. This will also be the case for other Twitter handles, including @WhiteHouse, @FLOTUS, @PressSec, and @VP. Individual official accounts, like @KS44, will transition to NARA and continue to be accessible for the public to view as an archive of all tweets sent by White House staff."

Will be interesting to see how many followers actually stay with the handle or even how many migrate to it.