Our team is in the process of planning legacy ERP replacing project. Unfortunately it is quite custom, so the decision was taken to develop own one. Legacy system has a lot of inbound integrations (point of sales, backoffice) and outbound (finances, etc.)
While we do understand how to develop the system, it's quite hard to plan and estimate implementation, integration and deployment phase.
for example: It's very risky to plan go live just over weekend, since it's 24/7 business. Probably we need to have legacy and new systems working in parallel. If so, which one would inbound clients (e.g. mobile apps) connect ?
Does anyone have such kind of experience ? What approach should be used here ? I would be grateful for WBS example.