Well, a 100% vote will use 2% of your remaining voting power. Let's assume that a 100% upvote from you pays out $0.20. If you change your vote and vote with 25% strength, then you will only use up .5% of your remaining voting power, but your upvote will only add $0.05 to the rewards. Hope that helps :)
RE: Reactions to HF 19 - Linear Rewards, Vote Strength, Changes in Payouts, Self Voting, etc.