In 20,000+ ebay sales, I've switched packages 3 times. Each time spurred me to tighten my procedures to avoid a repeat.
It's always a major headache, but I've found that most customers are very understanding if you catch the problem (or react, if they catch it first) very quickly and make the resolution as easy for them as possible. What's important is making the situation right for the customer with the minimum inconvenience to him or her.
In each instance, I've asked the recipients if they would be comfortable with my giving their name and address to the person who has their package. When they've agreed, I've paypalled each person the amount of the shipping from their address to the right address plus delivery confirmation with a couple of dollars added for their trouble.
I think it would be quite unlikely that a person would keep the incorrect item, but if they did, I'd try to keep in mind that my own carelessness created the problem and blame myself rather than the customer. Since I sell mostly antiques and collectibles, the chances that a customer would WANT the incorrect item are less than someone selling in a smaller niche.
Having customers return the item to you for remailing would not only greatly increase your mailing costs, but would also build in a delay that would be unacceptable to most customers.
It's been my experience that customers usually realize that accidents happen and respond positively.