Just redeliver the video assets you want to replace. You can reencode and redeliver assets at any time. Use the same vendor ID you used for initial delivery.
Redelivered assets will automatically overwrite the assets currently for sale on the iTunes Store. Allow time for processing before the new assets become available, just as you would when delivering assets for the first time.
It can take up to 30 days for a redelivered video asset to become available on the iTunes Store. To see if there are QC (quality control) issues for your assets, check My Movies.