Sounds more like a large refactoring than a rewrite. Or at the least you already has a service-oriented architecture with a well-defined interface that allowed the rewrite to be sane.
My rule of thumb has always been to break things down into the smallest discrete tasks possible. If you stay on top of your architecture this almost never turns into a total rewrite, but sometimes there is no choice but to get your hands really dirty in a major refactoring. In these cases it's critical that each escalation is well-justified and not just trying to capture more "low-hanging fruit" that the dev team is brainstorming in high-level discussions.
My rule of thumb has always been to break things down into the smallest discrete tasks possible. If you stay on top of your architecture this almost never turns into a total rewrite, but sometimes there is no choice but to get your hands really dirty in a major refactoring. In these cases it's critical that each escalation is well-justified and not just trying to capture more "low-hanging fruit" that the dev team is brainstorming in high-level discussions.