I let people change their picks for the following reasons:
I lock voting at the start of the first game of the week to make administering / tracking easier. This way I know no one made a pick after a game started.
Since voting does get locked at the start of the first game, if someone is late, I don't want to penalize them for all 4 games of the week. If you miss a full week of picks, very hard to remain competitive. I'd rather more people continue to play than not.
Since I let people who forgot to get their picks in still vote after voting is locked, it only makes sense to let people change a pick with the same rules.
If I didn't allow this, more people may wait until later in the week to make their picks, then forget, then miss completely, this again can lead to a decrease in participation.
This option is open to everybody, so I don't see how it is unfair.
Unrelated stat, but I find it interesting. Most years there are a lot of people who play the first couple of weeks, then trail off. This is for any number of reasons. People forget, fall behind, get busy with summer, who knows why. This is the first year we had 41 people in week 1, and while a. few people missed a week here or there, we still have 41 people playing this week. Good to see.