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.
I guess if someone changes his/her behavior fully and start contributing good stuff to the forum then most DT members who're still active on the forum will change their reg tags to neutral and that won't impact a user's experience at all.
There's chance that some of the DT members might remove the feedback that they have left when they see the good and contributing behavior of that member.