import praw
import sys

r = praw.Reddit(client_id='your_client_id',
        client_secret='your_client_secret',
        user_agent='blah',
        username=sys.argv[1],
        password=sys.argv[2])

subreddit = r.subreddit('your_sub')
for flair in subreddit.flair():
    user = flair['user']
    flairText = flair['flair_text']
    newFlairText = flairText.replace('g','b')
    print('Updating /u/%s\'s flair. Was \'%s\'; now \'%s\'' % (user, flairText, newFlairText))
    subreddit.flair.update([user], text=newFlairText)