It's not X days, it's 5 minutes...
(In principle, this question belongs on meta.SO, but I think it's worth having an answer here that doesn't require people to go looking over there.)
As far as I can tell, this answer of Jeff's (one of the SO founders, though no longer part of the company) is still the official reason: they don't want people to downvote a post to make it look bad, post their own answer (which will look better because it's not -1), then later take the downvote away.
There are a number of questions about this on meta.SO, but despite some good arguments against locking votes after 5 minutes, SO Inc. thinks it's fine the way it is, so it's not going to change.
There is a laborious workaround: edit the post then change your vote. But that only works if you have enough rep to edit w/out your edit being reviewed.
So basically, don't worry about it. Since Iain's answer is the only one with a downvote, it's obvious that that's the vote you'd like to change. But it's just one downvote and since he gave an answer you'd ruled out your downvote it arguably appropriate and - unlike some SE sites - I don't think SF has people who go nuts about every single downvote.
TL;DR: Don't worry about it, just move on.