Finally, forums are back online and I can post an answer.

In WS_FTP Pro version 8, you need to use Tools / Synchronize menu command. This will open a separate sync utility. There you just need to select the local folder in the left box, and the corresponding server directory in the right box.

Then just click Execute button and it will do all necessary trasnfers. If you haven't change anything, it will display:

* Synchronization completed - No updates necessary.

Otherwise, it will start transferring new files.