Without any research I have used Thunderbird as a BRIDGE to get this done. But before we start let's find out more about you. Some owners are going to want every scrap of data, all emails in same folders and well, perfection. For those I strongly suggest they hire a computer consultant and write out a contract.

Again, I've found it easier to use bridge products when the apps don't directly sync or export and import. I'm answering this way because you would have checked into sync or export/import already.