the only way you loose quality is if you are sending the video as a message, that is what happens with mesenger, whatssap and instagram, if video have bad quality when you send the message will look ultra bad, but when uploading the video to their platform is different there is not need to downgrade the video, they will adjust it to their format 1080 × 1920 9:16 I don't see the point to upscale if tiktok will lower the resolution again, this will make sence if the video you downloaded had bad quality you upscale it to make it a little bit better but if video is already HD just upload it as I said at the end video will be downgraded again to post it.