If you want to just replace the grease you have a couple of options. First though, you need to make sure that you have more grease on hand to replace the old stuff.
As you add oil to the mix it gets thinner. So you can add oil, take it for a short ride to heat up the lube, than drain and fill with more oil. It will probably take several fill-ride-drain-refill cycles to get most of the grease out.
When you get a drain that flows easily out the hole, you can be confident that you have gotten most of the grease.
Go ahead and refill with grease and you are good to go.
The swap to the sealed bearing is not that hard as long as you take your time.
If you want to make the switch, send me an e-mail and I will give you the benefit of my hard earned experience from opening my transmission up this past spring.