How to upscale an image without affecting pixel aspect ratio?
Popular cheap methods are:


Letterboxing

Cropping

A combined method: cropping, overlapped by the letterboxed image



The combined method works surprisingly well in videos, and people are quite accustomed to viewing such videos.

Photoshop and others also have smart content-aware scaling/resizing algorithms that re-organize or generate image content to the new image size. I don't have PhotoShop to demonstrate it with, but here is a video from Adobe.