I've been investigating this further and changing the bearings is not a trivial matter - especially if your throttle shaft is brass. I'm in a discussion with Keith (dcoe_tuner) on sidedraft_central and he recommends leaving it alone if there is nothing else wrong.
For those who are thinking about it (for example, you are rebuilding your carbs or want to upgrade as part of a larger carb project), he recommends still using the leather seals since the bearing's seals are for grease and not air. Also the adjustment of the throttle butterflies in the throat is very important and not trivial.
There is always more to learn!