Ok I found it out and if anyone ever need this here is how you can do it.
1. Go into net.minecraft.client.gui.FontRenderer.java
2. Find this function "func_175065_a" or the "drawString" function.
3. Add this code before the "GlStateManager.enableAlpha();"
if ((Minecraft.getMinecraft().getNetHandler() != null) && (SomeThing == true))
{
if (p_175065_1_.contains(Minecraft.getMinecraft().session.getUsername())) {
p_175065_1_ = p_175065_1_.replace(Minecraft.getMinecraft().session.getUsername(), "NewDisplayNameForTheClient");
}
}
4. Incase you have a empty name or nothing is in the Chat when you write something add this below:
if (p_175065_1_.contains(Minecraft.getMinecraft().session.getUsername())) {
p_175065_1_ = p_175065_1_.replace(Minecraft.getMinecraft().session.getUsername(), Minecraft.getMinecraft().session.getUsername());
}
5. So the complete Code would be:
if ((Minecraft.getMinecraft().getNetHandler() != null) && (SomeThing == true))
{
if (p_175065_1_.contains(Minecraft.getMinecraft().session.getUsername())) {
p_175065_1_ = p_175065_1_.replace(Minecraft.getMinecraft().session.getUsername(), "NewDisplayNameForTheClient");
}
}
if (p_175065_1_.contains(Minecraft.getMinecraft().session.getUsername())) {
p_175065_1_ = p_175065_1_.replace(Minecraft.getMinecraft().session.getUsername(), Minecraft.getMinecraft().session.getUsername());
}
~ MisterQuatsch