You can simply replace the old version on the server, keeping the file name the same, amending the duration and 'length' in the feed, and not changing the 'guid' tag. New subscribers will receive the amended version, but existing subscribers who have already downloaded it will not.
Or, you can delete the entry from the feed, replacing it with the amended version as if it was a new episode, complete with a differeng 'guid' tag and a different file name. Again new subscribers will obviously see this one, but existing subscribers will find themselves downloading the amended version in addition to the old one. Obviously you can't remove the old version for people who have already subscribed to it.
In both cases the Store page will simply show and play the new version.