Cutovers are hard, to be sure. Ideally they should also be short (the time time a service undergoing mitosis spends talking to the old and new locations should be measured in days or hours or less).

Don't choose general data access patterns for the infrequent occurrence of cutover. Cutover is when you break a few rules and then immediately stop doing so. Build for everyday access patterns instead (which should be through the API of whatever owns the data--SQL is a powerful language and a really shitty API).