Dear Jerry,
Let's say an user came to your site with tracking code = Email. It is being captured in s.campaign variable with the expiration of 30 days.
Next day the user came to your site via Direct and submitted the lead.
If you go to Tracking Code report, the lead will be attributed only to 'Email' because, the expiration of the tracking code is 30 days. Unless otherwise, if the Tracking Code is replaced, it will attributed only to 'Email' till the expiration.
But if you go to Marketing Channels,
Under Last Touch, it will be in Direct. Under First Touch, it will be in Email.
But sometime, we do not want the Organic to replace our last touch, if so, we can disable the override in the 'Marketing Channel Manager' : Below screen grab.

Thank You
Arun