What does "uva uvam vivendo varia fit" Mean?

The Latin phrase that appears on the Hat Creek Cattle Company sign in “Lonesome Dove” is a garbled corruption, and there’s no direct translation. It derives from the scholia to Juvenal 2.81 which cites the proverb “uva uvam videndo varia fit” This means something like “a grape changes color [i.e., ripens] when it sees [another] grape.”