4.3.6.4 Satellites Out of reach if disk space runs out. Operational 24 hours a day, receiving time-stamped messages from all over the world. Future dates will cause malfunctions before Doomsday. 4.3.10 Telephone switches Suppose calls in progress during roll-over are never billed. Stand-alone systems are relatively easy to test and modify.
Networked systems require synchronous modification, adapters or bridge software.
Testing is much more complex to plan and execute. General knowledge on migration strategies helps. Multiple parties involved, so it may take some time. A difficult issue. You need to know what systems you have, what tools there are, and how these match with your systems.