Ah. In that case, what OP said is indeed correct with a small addition, the curator actually ends up spending more mana as not only does the original vote cost mana but re-voting also spends mana.
So the curator would end up spending 50% more mana changing a 100% vote down to a 50% vote while also reducing the rewards they would have received.

For example:
A 100% vote is 2% mana, 50% vote is 1% mana, changing a vote from 100% to 50% would half the rewards you'd get and give to the author and you'd spend a total of 3% vote mana.

It's very inefficient to modify votes but it's often very rare it needs doing honestly.

Probably more likely to happen on very large accounts where they could accidentally over reward a post. :D