Quote by novetan:
"My question was "do we need to renew the grease"."
Yes. Grease becomes contaminated by dirt and moisture over time and use. It also degrades and breaks down. The question is, "How often should I re-pack my hubs?". I would suggest no more than a two-year service interval.
Hubs that receive zero use can still have the grease dry out in a few months of very hot storage conditions.
"The video titled "hub overhaul." Doesn't show us which part need degreasing and greasing."
The bearings. Cups, cones, balls / rollers, cages and seals. Freehub cassette pawls and ratchet surface (very light lithium grease is my choice for that area).
"Anyway. it does seems many parts need dismantling. I wouldn't dare to mess it up."
The actual hub bearings should be removed and once the axle is pulled and seals removed, they simply drop out. The cage installs in only one direction and only a complete moron could mess it up.
The inner roller/needle bearing could be serviced in place While you are in there it is only a little more difficult to press out the shell, thoroughly flush and clean the bearing, then press back in place. However, this does require investing a significant amount of money in dedicated tooling and yields no better repacking results.
Personally, I am of the opinion that anything pressed into an aluminum housing (the hub shell in this case) should never be pressed out unless absolutely necessary...clean and repack in place if possible. Remove the bearing only if replacement is required IMO.