the metal "bushing" that goes in between the bearings is there to take up space, and can be replaced (more cost effectively) with grease.
The wheel needs to be completely filled with grease upon install, and will take over a 1/2 tube per wheel, but at 3 dollars a tube (what I pay these days on this side of the pond), it is less than a new metal bushing each time.
You could reuse the old wheel's "bushing", as they don't usually 'wear' out, but then you have to knock out one of the old bearing to get to that bushing, which may cost more (at 1.50 to 2 dollars a minute {90 to 120 an hour} labor cost currently what most forklift companies are charging around here) than new bushings or the grease.