Say I was soft reseting Heatran. If it was shiny the next time I battle, will the overworld sprite change? For those who don't know what overworld sprites are, the Pokemon image following you in HG/SS is an overworld sprite.
No, the sprite will not be shiny + because you haven't encountered it yet.
The sprite for encountering the pokémon won't be shiny, but the sprite of a shiny pokémon following you will be a shiny sprite.