A forklift upgrade refers to any technology implementation that requires major changes to the existing IT infrastructure. Forklift upgrades may be the result of relatively minor enhancements that cannot be implemented piece by piece because of the age of the existing system. In this case, much of the hardware and software may need to be updated simultaneously, creating a job so large that it requires a metaphorical forklift to carry it out.
Techopedia explains Forklift Upgrade
The term is believed to have originated when the individual components of an IT system were much larger, and may have required a forklift to haul old components away and bring new components in. In modern terms, a forklift upgrade usually refers to a software change that requires extensive hardware updates or a hardware update that requires all new software. Forklift upgrades are most common in organizations with many legacy systems that make new implementations trickier as the surrounding systems age. With IT infrastructure, the choice seems to be between regular investments over time or the occasional (and expensive) forklift upgrade when current technology has raced too far ahead.