Upconverting HD footage to 4K will not improve its quality when played on an HD system. The quality will remain the same as the original HD version. This is because upconverting, also known as upscaling, is a process that converts a lower resolution signal to a higher resolution, but it does not improve the actual quality of the footage. It simply scales the image up to fit a larger screen, which can sometimes result in a softer or less detailed image.
When you upconvert HD footage to 4K, the process can introduce artifacts such as pixelation, blurriness, or jagged edges, especially if the footage was not originally shot in 4K. Additionally, if the footage was not shot with a high enough frame rate, it may appear choppy or stuttery when played back on a 4K TV.
Therefore, if you want the best possible quality, it's best to keep the footage in its original resolution and frame rate, rather than upconverting it to 4K. This will ensure that the footage looks as good as possible, without any loss in quality.