Targe can be changed by DT if the person that was tagged have made a good impression for the reason that lead to getting a tag, the only reason that can lead to tag not being removed is if the DT member is no longer active on the forum or if the member took the tag as a personal matter. Some DT can be biased and use their position as an opportunity to oppress other who are not DT. On this forum, I have seen DT drop tags on each other over personal mattersmatters, but by right if someone is tagged for spamming and the person later change, the tag should be corrected.
Tags can only be removed or revised by the user who gave them. DT members cannot remove tags given by another user. Retaliatory feedbacks are a wrong way to use the trust system, I have one from a user I gave negative tag sometime ago. You can tell these biased feedback from the accurate feedback through the reference thread attached to the feedback.
Yes, I know tags can only be removed from DT member that gave it but in the sense of being biased, some DT take their position as an opportunity to oppress others and even if the member that was tagged have changed their ways, the tag will not be removed by the DT member that gave the tag. I see the DT system as a system that was enacted to correct and not to oppress.