Our friends at Tricycle will push through a zero-based karma system (no self-voting possible) sometime this evening. At present this will only cover future posts/comments - they may go back and revise history some time in the indefinite future (or not), but apparently that would be overly complicated for now. We'll see how this works.
You seem to be thinking of things from the point of view of 'I don't want to suddenly find I can't downvote', where I'm thinking of it from the point of view of 'what is likely to lead to the most informative karma scores'. I don't think you're wrong given where you seem to be coming from but I'm not looking at it from the same perspective.