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.
If you reach the limit and encounter a post you really want to downvote you can always un-downvote a previous comment that you dislike less. That would be easier if the 'disliked' link showed comments as well as top level posts.
Yes, that's an obvious solution, but placing yourself in that situation is unnecessary, and implementing the amendment you suggest is a waste of time where the situation could be avoided in the first place.