Allow me the ability to change my vote, simply because something brought to my attention later may cause me to change my mind.
I can change my hall of fame vote, I can change my awards votes, but the most important votes I get, the approval or veto of a trade, I get all of 24 hours to make a vote that is locked in stone should I want to ever change it. Its happened maybe 1x or 2x in however many seasons I've played, but even if I've never been changed my mind on a veto, there may be a time in the future where I think it neccessary.
I don't always know if I'll have time in that 24 hours to come back and vote, so I have to make my decision in the 10 minute window I might have for that day, but what if things change and I do get a chance to log back in. What if somebody brings up a valid point that wasn't obvious at first and makes me reconsider? What harm is there in allowing me to change my mind if I find a compelling reason to do so?
The only draw back is the programming time and server space needed to allow this process to happen. I'll assume those are minimal in the grand scheme of things unless site staff tells me otherwise. Its not in the top 10 things I'd like to see improved in this game, but its a small change that may make things more user friendly.