Should you plan to run Dynamics GP at the same time as your old system for a period of time? The answer can depend on many variables. Some companies will absolutely insist on it and will not be dissuaded. In that case, you really do not have a choice. However, most companies will listen to the advice of the implementer on this and you should be prepared to discuss the pros and cons.
It will help identify any missing functionality, workflows, and reports that have been left out of the planning.
It will give the opportunity for additional time to address any missing items identified.
It will give the company's management a sense of security. Running a new system concurrently with the old one will feel like a more conservative, safer decision to many people.