Rules could support the change of Categories if they don't interfere with the automated category sorting algorithms already in place. Otherwise, the introduction of a simple rule could mess up a lot of established workflows. I therefore suggest that category rules should only be applied after passing through the default category filters. (e.g. could be applied only to rules already sorted into a specific existing category) For instance, I get email from an online merchant all delivered from a specific email account, while some are order confirmations/receipts and other messages are promotions/sales that I want to sort into a lower priority category because they are more likely to end up in the trash.