In a responce to a post on the US Battle.net forums Jeff Kaplan has said that the development team are looking into allowing the user to change on the Hero selection screen, rather than having to exit the match and switch to the Hero Gallery.
In the post he said:
We also would like to give you this option. Our design for it allows for changing skins during “Assemble your heroes” but not after that phase. We want people to be able to change skins before a match but we also don’t want people changing skins mid-match.
He also added that most likely players would only be able to change this at the very start of the game, to prevent confusion and to stop players sitting at spawn trying to figure out which skin to pick while the rest of the team attempt to play.
He goes on to say:
We have two reasons for this:
1) Your team needs you. Go help them
2) We don’t want to add confusion to the “read” of the game by having people constantly changing appearances during the map.
And then finishes by pointing out that this is a low level feature request, meaning that it may get done, at some point, but is not exactly important to the functionality of the game.
This feature has been on our list for a very long time and is something we want to make happen. It usually gets deprioriritzed for other things (such as the Season 2 competitive changes). It’s something we really want to do though.
We are looking forward to being able to change our skins at the start of the game, there are so many colours of Mercy Skin for us to chose from!