Questions on upgradings clients

Hello,

I'm upgrading SEP clients from 12.1.4013 to 12.1.5337

One of my Client groups in SEPM has almost 100 clients and I'm worried about grinding the network to a halt by suddenly performing an "upgrade clients with Package" from SEPM. My plan was to create a temp group and move them across in batches to mitigate the problem. I have a few questions:

• How big is the 12.1.5337 package?
• If an installation package is assigned to a client group in SEPM, will the clients start downloading it as soon as I move them into that gorup?
• How can I tell when the clients have finished downloading the software from SEPM?
• Once the download has finished, can I move them back to their original group, or do I need to wait for the installation & reboot as well?

Thanks