Converting a Video from 1600*1200 to 1920*1080
Using ffmpeg,

ffmpeg -i input.mp4 -vf "scale=1920:1080:flags=lanczos,setsar=1" output.mp4


This will stretch the video to 1920x1080.



ffmpeg -i input.mp4 -vf "scale=-1:1080:flags=lanczos,setsar=1,pad=1920:1080:(ow-iw)/2:(oh-ih)/2" output.mp4


This will keep the aspect ratio but place black bars on the left and right.



The flag in the scale filter determines the scaling algorithm to use. lanczos is the best available, in general, for video.

The setsar sets the pixel aspect ratio. Advisable to keep this square (1) unless you know what you're doing.

In the 2nd command, the -1 tells ffmpeg to preserve original proportion when scaling height to 1080. The pad fills out the canvas to 1920x1080 and centers the scaled video within it.