Sealed bearings are "permanently lubricated". Yeah, right. Most have seals that aren't too hard to get off. But these cheap ones I get from Farm & Fleet are a real mutha. Couldn't pull seal out with a pick tool, the metal crimped edge was too strong. I had to drill a tiny hole in the seal, screw a metal screw into it and rip it out, being careful not to damage the bearing, of course. That kinda deformed the seal, but it was easy to re-flatten and press back in.
Since I drilled through the metal seal, I thouroughly cleaned the bearing with solvent to remove all old grease and metal shavings - then repacked and resealed.
Anything can be done if you are determined (and hate spending money).