Big bang replacement of existing business system

1. Don't.
2. Almost certainly underestimated the complexity of the system you want to replace.
3. The users have learnt to live with the existing system.
4. The existing system includes endless Excel and Access things the users have knocked up and love.
5. The existing system will improve while you work on the shiny new replacement.
6. Building a new system from the ground up is exhausting.
7. Near impossible to manage user and management expectations of the new shiny system.