A Company wishes to change from an old computerized system to a new computer based accounting system. Explain how and why both systems should run parallel prior to the change over to the new system

How and why both systems should run parallel prior to the change over to the new system.
How

The client should continue to process the information using the old system while at the same time implement the new system. The output from the new system should be compared with the output from the new system.
Why
i. Parallel running assists in staff training. The users learn about the features of the new system and compare this with the old system
ii. Assists in testing the new system.

Allows for the systems developers to make any necessary changes to the new system before being fully implemented.