Final answer:
System upgrades are developed during the maintenance stage of the SDLC, which is the last phase where continuous improvements are made based on feedback and advancements.
Step-by-step explanation:
System upgrades are typically developed during the maintenance stage of the System Development Life Cycle (SDLC). The SDLC stages proceed in the following sequence: requirement gathering and analysis, system design, implementation or coding, testing, deployment, and the final stage of maintenance. In the maintenance phase, the system is in use, and any necessary upgrades, updates, or fixes are developed and implemented to enhance the system's performance and address issues that were not apparent in the earlier stages. This is the phase where a system is refined and continuously improved upon, based on user feedback and technological advancements.