There shouldn't be any problem updating but there is always the risk of losing everything, in this case a backup is highly recommended. If you attempted to update via OTA (Over The Air) then there is a very minor chance the data will get corrupted. Alternatively, any firmware update done through Kies might delete all your apps, videos, settings, etc. If the update completes successfully, attempt a backup of the phone as soon as possible. You can do it with Kies application, the official product for Samsung S phones.