What happens if I turn on iCloud Drive sync while there are already different files in the cloud and on the local drive?

I have a MacBook where the Documents folder used to be synchronised with iCloud. Recently, the system was reinstalled, the synchronisation was not re-enabled and changes were made to both iCloud Drive and local Documents folder. I would like to enable synchronisation again. Is this likely to delete or alter files?

For context, the local Documents folder is almost empty at this point as most documents live in the iCloud Drive. Clearing the local folder before enabling synchronisation is an option. I just would like to be sure that the synchronisation will not delete everything in iCloud Drive to mirror the current local Documents folder.