I like easy tools!
From this article, one should remember that from any project, someone should take care of:

1) Interfaces
2) Communication

Meaning:

When an interface needs attention, there must be two teams (system and design architects probably) talking!
Therefore, a sort of matrix that a) inventories the relationships between the legacy system and the new one b) interfaces that are modified, the ones that won’t c) unidentified interface.

The matrix will show you the any lack of interdependence between the providing and receiving teams.
Alignment is best… as always!