There really should be some sort of penalty. If you vote 100% and then I downvote because I think that is excessive, and then you reduce your vote to 50%, I've still used up my downvote mana. Similarly, you might make a big vote giving someting visibility and other smaller voters might follow on expecting the post to trend, generating curation rewards. Then when you remove/reduce your vote, the chance of trending is reduced and the later voters may regret their votes (despite potentially gaining relative curation reward share).

I think the inherently sequential nature of voting is pretty important and there needs to be sufficient penalties while allowing people to change votes when actually necessary. An exception could be if there are no subsequent votes (you are still the last vote), perhaps it should be possible to change vote without any sort of penalty (a misclick exception). But that will be an edge case and even very unlikely to ever happen if the user base is large enough with many votes coming in all the time.