A forklift upgrade is the complete overhaul of an IT infrastructure.
A forklift upgrade often involves ripping out and replacing outdated technologies. The term forklift upgrade has its roots in the early days of computing when mainframe computers and public branch exchange (PBX) telecommunication switchboards were so big and heavy that it literally took a forklift to move them when they needed to be replaced. (A forklift is a small vehicle that's used in a warehouse to raise, lower and carry heavy loads.)
The term forklift upgrade often has a negative connotation; projects that involve incremental improvements are usually less disruptive and less expensive than rip-and-replace projects, so most administrators try to avoid forklift upgrades whenever possible. Sometimes, however, an administrator may have no alternative and a forklift upgrade may be the only reasonable course of action, especially if a vendor goes out of business or stops offering support for a legacy product.
When it becomes evident that an in-place upgrade is not cost-effective, the term IT transformation may be used in place of forklift upgrade. Transformation has a more positive connotation, implying careful thought and strategic planning.