Converting a Landscape (1920 X 1080) texture into a Portrait mode (1080 X 1920) texture

Just make two layouts, one with both textures stacked for portrait, the other with one for landscape.

You could even leave them both in the scene with some code to enable one or the other based on screen orientation.

Along with that, get your anchoring / scaling correct and you’ll be golden.

Here are some notes on UI Anchoring, Scaling, CanvasScaler, etc: