Downloading changes online is not suitable for transferring mass data but is instead designed 
for inserting new measuring points or measuring point changes into a project.
Permitted actions for downloading changes to the OS
An overview of the points where changes can be made can be found in the section 
"Downloading Changes to a Project" in the "SIMATIC Process Control System PCS 7 Operator 
Station" (https://support.industry.siemens.com/cs/ww/en/view/109485970) manual.
Changing projects in process mode
11.1 Downloading changes online
Compendium Part A - Configuration Guidelines (V8.2)
290 Configuration Manual, 08/2016, A5E38166869-AA