Google Account Conflicts

We are planning to pilot a Google Apps domain this spring. One issue to tackle is conflicts between the new school Apps accounts and existing personal accounts. Many users have set up personal Google accounts using their email addresses. When we launch the Apps domain, these users will see the following message the next time that they log in.

It will be important to communicate well about this transition before the launch. Once users clear the above hurdle, will they be able to migrate their content or favorites into their Apps accounts? If not, how much inconvenience will this cause them? At worst, active users will suddenly have two accounts, an old account with a new address and a second, new one with their address. How many people will find themselves in this situation?