Is grease purging from the seals? If not, remember that the grease cavity should have no more than 30% grease by volume. You also take the chance of mixing incompatible greases.
The grease that's in there should last a good long time, as long as the seals don't wear out.
BOTH bearings are suspect? If the bearings truly aren't running smoothly, then the damage is done and they need to be replaced.
Im the third owner, its 15 yrs old with 225K miles.. i dont believe everything i see on youtube but its a decent idea IMHO the guy actually says "good for another 100 thousand miles" after he shoots the grease in..
I think its a semi-sound idea for 'preventative maintenance'... the bearings are tight and dont roar, just saying they aint OEM fresh.