Hey @RobinOscar, welcome to our Community and happy Friday!
During this upgrade, you’ll continue to have access to all of your content normally. Edits to your files will only sync to other devices when this is complete though. After the move is completed, Dropbox will ensure your files are up to date.
If you find any files or folders that have been set to be online only during the upgrade, you can make them available offline again, simply by using these steps.
Let us know if you have any other questions!