I think that's not what Helena asked. She asked about what would be happened for a member previously red tagged by a DT member(Members who handpicked by theymos) who lost their credibility even after that member red tagged. Not about the trust rating put by untrusted users which we don't consider normally.
If a current DT Member lost credibility then it will become an untrusted feedback to scammer and the red tag will get erased from that scammer profile.(If scammer is tagged by that single DT member only.)
I am not 100% sure, but I think Pharmacist lost the DT status for few days.
If they were trusted and they lost their credibility -just if that' s the case- then, what happens with the trust they give to others?? Sorry, my brain is just exploding here. 
Actually, you're making a good point here and I'm thinking red trust given by particular DT member should be removed (Not other members given) if that DT member lost their credibility. b) Even DT member lost it credibility and accusation is proved but it does not mean that user lied earlier in every occasion. User feedback should remain there.
Untrusted -ve feedback are also important while making a deal. Might be DT did not find enough evidence to tag or the scam accusation did not get attention of DT.
(theymos didn't say anything about trust 
)
)Read the whole post "https://bitcointalk.org/index.php?topic=211858.msg2221664#msg2221664" here.