When donating for a higher rank, set votecount to the minimum number of votes to reach that rank.
This will allow for voting up to higher ranks starting from the baseline of the rank you donated for. 

The only downside that i can see is if you had close to your monthly reward like having 80 votes and only need 10 more then getting set to 180 votes and having to wait a month for 90 more votes. So you would lose your shulker box reward.

The counter for the weekly and monthly reward dont need to reset. If it was 10 votes it can still be 10 votes.

How hard would it be to make ranks more similar to rewards? This many votes to the next rank kind of thing. Lightfoot to Peddler (12 to 42) is 30 votes. If you upgrade your rank, the number of votes isn't reset, but the number of votes to the next rank is (in the example, now 30 votes required to rank up when you buy Lightfoot). Make it explicit that votes made toward the next rank are gone, you're starting from the base difference in votes, just like rewards. There's already a "next reward" system in place, could it easily be adapted to include ranks as a "reward"? If not, I don't believe a system that reset someone's votes to 500 when they bought Elder would properly serve the community.