Hi, and welcome!   First; it's more correct to ask how to change from Unova (the region in Black/White) to Johto (the region in HeartGold/SoulSilver) since the game mechanics are the same in both places.   Second, Johto isn't implemented yet, so you can't go there. The HGSS ROM is to display pokemon following you in the overworld, since that was a feature in those games.   Finally, an actual answer to the question: To change your region, you need to get 4 badges in