the single account will get a sort of benefit by only being charged the delta for an upward adjustment

That's not a benefit relative to multiple accounts. With an original vote of A and a new vote of B, the voter is charged for A+B-A. But with two accounts, the first account would vote A and be charged for A, and the second account would vote B-A and be charged for B-A, for a total cost of again A+B-A.

Maybe that's too complicated

I think it is because I'm coming at it from the perspective that vote changing should not be normalized. There are just too many ways in which it complicates and undermines the sequential nature of voting (where voters act with knowledge of previous votes). IMO vote changing is useful in certain exceptional cases such as mistakes or new information about the content, but mostly people just just take more care in voting without expecting to be able to change it with no or low penalty.

I do agree with not incentivizing account splitting, which calls for penalties being small or zero to the extent they can be circumvented with multiple accounts (as above).