Change the Resolution

[Sound Master]

I have a 1080p video, and I want to lower it down to 720p. How do I lower the resolution of videos and images, and bring it back up again without losing quality?

[ProWo]

That is not possible. To change the resolution you have to re-encode; any re-encoding will result in a loss of quality unless you encode with a lossless codec, but then the file size is so large that this option is useless.
But you can re-encode with little loss of quality.

[Selur]

The problem isn't the loss due to compression, but the loss due to lowering the resolution and thus throwing away information.
Downscaling and later upscaling (in general) was possible, I would downscale everything to 1 pixel and only upscale on playback.

[johns0]

Just encode to 720p and keep the original video.

[pandy]

I agree with johns0, other variants are create 720p video, upscale it back to 1080p and subtract from 1080p source - you get delta video which should in theory keep all differences (loss) between 1080p source and created 720p video. or use something like AVC extension called SVC (or any other video codec supporting https://en.wikipedia.org/wiki/Layered_coding?useskin=vector )