TL;DR: users should be allowed to edit their existing area51 votes in-place when they cast a 6th on-topic or off-topic vote.
I'm a big fan of being limited to 5 on and 5 off topic votes per proposal. With that said, I feel the current UI is not optimized for changing your mind.
Perhaps it is cumbersome on purpose, trying to encourage users to think a bit more before casting their initial votes. If so, the topic of how to best encourage thoughtful vote casting is for another thread.
However, I often find myself voting for 5 on topic questions only to find a better example a bit later on. To get around this limitation, I either have to read all the questions first, then vote (infeasible and cumbersome), or I have to find the votes I already cast and remove one (time-consuming).
A proposed solution: when a user attempts to cast a vote when over the limit, instead of prompting them with text to remove some old votes, we present them with their current votes with the option to replace the new vote for the old one?
In this way, users can easily change their mind when they find better examples. This may actually encourage even better results, as I often find myself reluctant to change my votes once cast, even when I find a better example.