We would like to upgrade a very large project to 4.X At the same time we would change to https. My question is: when should I change http to https? As I have understood "Serve images from local server" would require to rebuild the content. The content will be also rebuilt after upgrade. Can I combine the both "rebuilding" to avoid rebuilding running twice? Rebuilt tooks some weeks on our server. And when it is better to change to https? Before upgrade, during upgrade (what step?) or after upgrade?

You might submit a ticket on this one. IPS may also help you on the upgrade from 3.x.

Aiwa, thanks! They actually will. I am now running test upgrade myself on the test server. Just to be prepared, that's why I ask :)

If you intend to use the "Serve images from local server" option, then I would recommend (1) change to https before the upgrade, and (2) toggle the option immediately after the upgrade completes (but you don't have to choose to rebuild the links in that case). This will, as you suggested, mean the issue is addressed with one rebuild instead of two.