Let's not forget these are complementary systems. It's just like having more signs on the highway. The old system is still there with the ability to leave feedback. So really there is no need to go back over everything in the past.
Positive - You think that this person is unlikely to scam anyone.
Neutral - Other comments.
Negative - You think that trading with this person is high-risk. You might also be able to add a flag.
The ratings are still there and moving forward the flags can be applied as needed.
A negative rating right now is completely useless and will be disregarded by the supermajority of the users (the same way that neutral ratings always have been). I'd actually advise against leaving them to save yourself the time and trouble; just skip straight into scammer flags.