I realize the ITIL importance of making closed tickets un-modifyable, as a system of record. But, as a part of post analysis and reporting on hundreds of past closed tickets, mining for patterns of support issues, etc., I would like to be able to add and edit the tags on long closed tickets. For example, if I have an epiphany and want to do something in my Zendesk to help me with something like, "Wow, we sure seem to have been fielding a bunch of upgrade issues with our installed customers over the past 3 months. I think I'll go try to tally those tickets into some kind of category..." ...just as an example. It seems that post tagging might do the trick.