Once the owner has been AWOL for 14 days you may replace him. Just should admin a message once the 14 days have passed and you have a replacement lined up letting them know that "so and so will be taking xxx franchise." They will handle the switch and yes it won't cost the new owner anything for this season.
If the 14 days haven't passed yet, I would recommend starting a search for a replacement now. That will give you time to be picky about who takes over the team. I would look for an owner who has a history of staying long-term in their worlds. It gives you a better chance of the owner staying for future seasons.