Nintendo has confirmed that the Joy-Con included with the new Switch OLED will be the same as the current controllers, meaning any Joy-Con you have now will work with the updated console.
In addition, every Switch dock is compatible with all Switch models, so the dock that ships with the OLED version can be used with a regular Switch, but, an update may be required.
Nintendo also states that “there may be differences with in game experience with some of the Toy-Con accessories from the Nintendo Labo series, due to differences in console and screen size between Nintendo Switch (OLED model) and Nintendo Switch. There may also be games where the game experience may differ due to the new capabilities of the console, such as the larger screen size.”