Yes, however, you should not try to reclassify messages while your email client is actively getting email. Find the messages in the history that you wish to reclassify, use the drop down box and select the bucket you wish to reclassify to and hit the reclassify button. If you are reclassifying a large number of messages at once, it can take a moment to finish. In v0.20 and later, the delay is nominal.