Wow, I'm disappointed in all this naysay on taking bearings apart to change the grease out. I just did this right now and would love to re-open this thread with some real facts and pictures.
I am a retired master mechanic, and am always sad to see a concept called "preventative maintenance" becoming a scarce thing, with instead "expected failures" as a result. As if anyone here has seen a super high quality grease inside even a Timken bearing? https://electricalfundablog.com/identify-bearing-number-calculation-nomenclature/
should make this post worthwhile. That's the nitty-gritty for just what goes into bearings, and trust me, the usual lubrication is a joke. Bearings aren't built to last forever, but you sure can help them get there.
Let's look at this grease found in most bearings, even high-quality Turkish units found on a high-end Audi. I just finished cleaning it out with chlorinated solvent-- use this so it doesn't attack the plastic cages inside. Then direct high pressure air through the bearing afterwards to remove the remaining residue. Do this outdoors or in a well ventilated area since this is toxic stuff.
As you can see here, the grease which is from 2008 has completely ceased flowing, and the bearings were completely dry. They had no lubrication. This is on a low mileage well-stored vehicle, and the price of these parts far exceeds the time it tooks to simply remove them and pop the seals out, clean them and regrease them and then expect an extremely long life. This was done as preventative maintenance while the intake valves were decarbonized (1st gen direct-injection engine carbon problem.) Seals can easily be opened without damage using a tiny flathead screwdriver pried from the inside.
Videos on how to do it are abundant, as high-end bearings for skateboards etc all are sealed units and people want to open them up to change the grease to something else etc.
As for the comments about grease mixability, yes it is true that white lithium grease will cause all sorts of problems with sodium-based greases, but if you're using something like Mobil 1 synthetic grease then you shouldn't have to worry, right? In any event, chlorinated solvent can be used to safely clean out a bearing without harming any of the races, without leaving any residue of any kind. Find it in an auto parts store as electrical parts cleaner or alternator cleaner. It is non-flammable.
Grease can be packed in, depending on the size, with your gloved palm and just push. It's readily apparent when the grease pushes out the other side, you've done the job well. Pop the seals back on with nothing but your fingers, and everyone can enjoy a bearing that will now last forever.
I like Redline CV2 grease, not really sure there is much better, but hopefully, my first real post to BITOG is a good one and not a firestarter. :P
Have a safe holiday!