Computer industry jargon. A
forklift upgrade is an upgrade to a computer system requiring a large amount of new hardware (enough, presumably, that a forklift might be necessary to deliver it). Often this can mean completely replacing old hardware with new; hence "upgrade" is somewhat of a humorous misnomer.
Typical usage: "You can add up to a gig of memory to this box, but after all the slots are full we'll have to do a forklift upgrade".