A lot of round trip workflows are still based on the original sequence reading different (colour corrected) versions of the media it originally sent out. The sequence itself doesn't change. Usually timewarps that were applied in the original sequence can still be accessed and changed. Text lines from that sequence remain as well, so it is perfectly understandable that your sequence frame size would remain the same. I'm not a Premiere Pro editor, but I would hope you can change your sequence size to UHD and continue working.