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 you will not be able to dissuade them. 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.
Running two systems simultaneously will provide the following benefits:
Help identify any missing functionality, workflows, and reports that have been left out of the planning.
Give the opportunity for additional time to address any missing items identified.
Give the company 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.