"Big Bang" means you stop using your existing system one day and your entire company goes live on the new system the next day.
The alternate approach is a phased, module by module, implementation - (I call this approach the "Rolling Thunder".
The problem with Rolling Thunder approach is that in an integrated system each module relies on information from other modules.
I strongly recommend big bang for the main modules (sales orders, production and purchasing). Other modules like human resources, document management, maintenance and even financials can be implemented later.
The critical step in successfully implementing the Big Bang is proper testing.
I recommend a conference room pilot approach where computer work stations are set up in a room to represent each of the major tasks of customer service /order entry, planning, goods-in, stores and finance.
At least 50 orders should be completely processed through all the modules and carefully reviewed. No company can afford to justify a complete parallel run and you will spend more time trying to reconcile the two systems including data entry errors, different reports, cut off dates and so on.
Provided the conference room pilots above have been run properly at all three stages, project team, management and users, the least risk strategy is to cut-over with a clean break from the old methods (Big Bang).
The conference room pilot increases everyone's confidence in the cut-over and highlights any short comings that need to be corrected.
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