Copy & Pasting from horizontal to vertical sequence leads to quality loss

Here's the problem:

When converting a horizontal project to a vertical sequence, there is no good solution outside of third-party tools. You'll have to accept either huge black bars at the top and bottom of the vertical image or image quality degradation due to upscaling using a less-than-optimal alogarithm. And rotating the image to fit the full frame will not solve the problem because the vertical image will be flipped sideways when viewed normally. Set to frame will still leave a horizontally framed image with massively reduced resolution (in this case, 1080x608, or sub-720p) with black bars both top and bottom. So Set to Frame actually reduces resolution due to trying to fit 1920 pixels into only a 1080-pixel dimension.

What you're doing is improperly upscaling a 1920x1080 image all the way up to 3414x1920 using an unoptimized alogarithm and then cropping off both the left and right sides of the image. Premiere Pro by itself does a poor job of upscaling images to begin with. (Remember, Premiere Pro works far better at downsizing than upsizing videos in its native scaling alogarithm.)

In other words, you've almost blown up a 1080p image into faux 4k by trying to somehow fit only 1080 pixels into a 1920-pixel dimension space-a "Where's the beef?" scenario in which you add more "bun" without any meaningful increase in the "beef" content, and in which the "bun" buries the "beef" taste even more.

Such a job definitely calls for After Effects' "detail preserving upscale" effect at the very least, or better still the third-party Topaz Ai video tools.

Do you have the  option in Preferences to scale to set? I think that is still there, and if so, would cause this. Scaling changes to the pixel framesize of the sequence. "Set to" uses the full original pixel dimension but sized to the chosen sequence.

@Christopher Shipe this is one good work-around. And it's effective. You can also try Instead of manually copying and pasting clips, duplicate your horizontal sequence and then right-click the duplicate to either use the Auto Reframe tool or manually adjust the Sequence Settings to 1080x1920. When reformatting, use the "Set to Frame Size" rather than "Scale to Frame Size," as this will maintain a direct link to the original high-resolution pixels instead of simply resizing the video container—it preserves the original image quality while automatically adapting your timeline to the new vertical aspect ratio.