Installation

Parallel

•  Old system continues to run while the new system is installed; When there is confidence in new system, old system is retired.
• Advantages: Accuracy of new system can be compared with legacy system; Can revert to old system if there is a problem ; Suitable for life/mission critical systems
• Disadvantages: All work is duplicated; May not have space for 2 sets of physical equipment; May result in data integrity issues

Direct (Plunge or Big Bang)

• The new system takes over from the old system immediately at a given time; Old system is retired immediately
• Advantages: No risk of data integrity due to duplication; Cheap…