Under the current system, if you remove a vote you forfeit all future curation rewards on that post even if you revote the post prior to payout. In other words, if you vote 10% on a post and later decide you want to reward it 20%, you no longer earn curation rewards.
This behavior is to prevent users voting a minimal amount on a large number of posts and later increasing the vote on successful posts to maximize curation rewards. By forfeiting all potential curation rewards when you unvote a post, this behavior is not attractive.
I do not believe you should be penalized to change your vote either higher or lower, outside of course the loss in voting power for the additional votes.
This topic has been discussed a lot in the past, and the original developers agreed it should be changed and planned on changing it on a future hard fork.
The way I propose this works, is by completely removing the forfeit curation rewards penalty. If you remove a vote, and later revote a post the time of your last vote is used when calculating curation rewards. This means if you vote after 15 minutes and remove it and revote it 2 days later, this last vote will be used to calculate your position in the curation rewards pool. This eliminates any way to manipulate curation rewards.
Under this change, voting power will still be used on all votes. So to vote 10% and remove it for a 20% vote would require 30% voting power. I also propose this change as well, only charging the voting power delta for new votes.
For example, if you vote 10% and later change it to 20%, you would be charged another 10% voting power, a total of 20%. If you voted 10%, and then later change it to 5%, you would still have expended 10% and the 2nd vote wouldn't refund any voting power. This is another change that was under consideration in the past and was well received.
There is also an active exploitation being used to take advantage of a side effect of the current behavior. When a user unvotes a post, their curation rewards are forfeit but still exist in the curation pool granting all curators more curation rewards than their current stake entitles them to. I know of at least one user that exploits this unique behavior.
This proposal recommends the following changes in a future (hopefully next) hard fork.