I started a long animation render (many hours) with the default Autodetect setting in Render / Performance threads. I have a quad core machine. Fearing overheating, about 1/3 into the render I changed the setting to Fixed and 2 threads. I assume this means using two cores and therefor half as much heat generated due to rendering. But does making this change actually have an effect during a render, or do I need to restart it?
Change this setting during a render process change nothing, I made a test to make sure.
You have to stop the render change settings and start again.
No problem if you render images but a problem if you render video files.
But you have a video until the frame you stop the render, start the render at this frame and compose the two videos in the blender video editor or other video editor. Change the name of the first video to make sure it is not overwrite from the second.