Overfilling a sealed bearing with too much grease can be just as harmful as underfilling a sealed bearing.
When a sealed bearing is overfilled, there’s no space for the heat to dissipate. As heat builds, the bearing can find itself operating at temperatures above what are considered normal [for the installation]. As the bearing’s temperature rises, the grease begins to break down due to churning. This churning causes the bearing’s temperature to continue to rise, which in turn increases the rate at which the grease breaks down. The tit-for-tat continues until damage to the bearing occurs due to an insufficient amount of lubricant to separate the sliding and rolling surfaces of the bearing.
If you feel the need to pack/repack a sealed bearing, place a rubber wheel on a drill and with one of the seals removed, use the drill to spin the bearing for 30 seconds or so. There’s no need to go crazy with the speed. You just need to spin the bearing long enough to evenly purge the bearing of excess grease. Reinstall the seal after the excess grease has been removed.
You can also purge the bearing of excess grease by hand. Just takes a little longer to get there.