"sealed" cartridge bearings are never truly 100% sealed, they have to allow gaps in the inner diameter of the seal/cone interface so an axle can spin. The grease injected in them does eventually degrade or get lost due to a number of factors, it just takes longer and they get far less intrusion from contaminants than conventional bearings...but it does happen. I've gotten seals off and cleaned/repacked cartridge bearings, but it's akin to micro-surgery, and very hard to do without crimping the seals and rendering them useless...I actually used a scalpel to pry the edge up...I'd only do it when there was no way to press out old and in new bearings in a unit. Some older ones have a very thin spring clip that holds the seal in place, these are easier to remove the seals from, but still not what I'd call "easy".