Funcom announced that downtime will precede the launch of Conan Exiles' expansion, Isle of Siptah, probably because this is the biggest content update for the game yet.
Isle of Siptah, Conan Exiles' biggest content update yet will launch on September 15, 2020, with the price tag of $19.99. It is going to add a new island to explore along with a pile of dangerous new creatures to kill and the whole shebang that follows such as new building sets and a gameplay cycle.
However, before all of these goodies enter their Early Access period, Funcom will be performing maintenance to ensure a smooth launch, which will cause the servers to go down on the same day at 10:00 AM CEST / 1:00 AM PT.
Unfortunately, there is no estimation on how long the servers will be down and players can only speculate for the time being. Console versions of the game will either remain untouched or have shorter maintenance since the Isle of Siptah isn't coming there before 2021.
That said, keep in mind that the PC version is Early Access so if you do venture to the new isle, expect a few things to be wonky and some bugs to crash your game or something, as opposed to straight-up trying to kill you.
According to the description from the trailer embedded in the article, the players will start there from scratch since there isn't much left to imagination after hearing that you get shipwrecked and "with nothing but the scraps on your back, you craft the tools you need to survive".
While not being able to carry progress over will rile up some players, it may be for the best to experience the new lands from scratch as there will be more objective completions to strive towards.