If you've logged that clips, then you should be able to just re-digitize the clips from the main camera. Right-click on the problem clips in the rack and choose Batch Digitize. The re-captured clips will replace the existing clips. Remember that you'll have to re-render the timeline afterwards.
If you want to do a comparison, when you re-capture go to the Media tab in the logging tool and change the quality from Normal to Better. This won't replace the existing captured clips, but make new clips under the same name. Now on the timeline you can change the timeline properties so that the desired quality is Better or Best (leave the minimum quality at normal) and the re-captured clips will replace the older clips. To see the older clips change the desired quality back to Normal.
If you imported the clips, or used EZCapture, then you'll have to replace the files in the media directories with new files of the same name. Note that you must use files that are exactly the same length as the originals.
Hope this helps.