A migration plan is only as good as the inventory beneath it. The surprises that derail migrations are rarely technical puzzles. They are an undocumented downstream feed, an application that turns out to support a critical business capability, an owner who was never consulted, or a system that should have been retired rather than moved.
Sequencing decisions are often made for infrastructure convenience: by data center, by server, by team. The business sees the result as a series of outages and broken interfaces, because applications that exchange data were moved months apart.
The planning work is to build a shared, trusted picture of the landscape first, then decide each application’s path and the order of the moves.