How do I replace vanilla blocks?

Not curently, No there is not. The only way is to create your own biome with the trees.

You Can Retexture Vanilla Blocks, But It's Hard To Untextured Them If you do because it will texture this block in every workspace...

So what you do is make a texture and then save it as the blocks code name

example: If you want to retexture Oak Logs, you would name your save log_oak and then import it into your MCreator (You Don't Even Have To Make A Mod Of It Just Import The Texture)

Now How Do You Get This Texture Out Of Your MCreator?

#1 Go To   C:\Pylo\MCreator\user\textures\blocks and delete the texture if it's there (it night not be)

#2 Go To C:\Pylo\MCreator\forge\build\resources\main\assets\minecraft\textures\blocks     and delete the texture if its there

 #3 Go To C:\Pylo\MCreator\forge\src\main\resources\assets\minecraft\textures\blocks

 and delete the texture if its there

#4 Keep Looking To See If Anything Eles In The "forge folder" has that texture, if it does delete it.

#5 The Texture Should Be Out Of Your Test Environment and Out Of Your Mod :D

Try this:
1st) make an empty GUI Overlay. (You will use it as a "Event Handler")

2nd) open code of it.

3rd) delete this part:
 @SubscribeEvent(priority = EventPriority.NORMAL)
    public void eventHandler(RenderGameOverlayEvent event){.....}
So you will have only empty class GUIRenderEventClass

4th) 

In the empty class put:

@SubscribeEvent(priority=EventPriority.NORMAL, receiveCanceled=true)
    public void populateChunk(PopulateChunkEvent.Pre event)
    {

        Chunk chunk = event.world.getChunkFromChunkCoords(event.chunkX, event.chunkZ);
        Block BlockA = Blocks.coal_ore; 
        Block BlockB = Blocks.diamond_ore; 
        for (ExtendedBlockStorage storage : chunk.getBlockStorageArray()) 

        {
            if (storage != null) 

            {

                for (int x = 0; x < 16; ++x) 

                {

                    for (int y = 0; y < 16; ++y) 

                    {

                        for (int z = 0; z < 16; ++z) 

                        {

                            if (storage.getBlockByExtId(x, y, z) == BlockA) 

                            {

                                storage.func_150818_a(x, y, z, BlockB);

                            }

                        }

                    }

                }

            }

        }

        chunk.isModified = true;
    }

5th)  on line Block BlockA = Blocks.coal_ore change Blocks.coal_ore to block that you want change

on line Block BlockB = Blocks.diamond_ore change Blocks.diamond_ore to block to that you want change BlockA

This method does not work anymore...

Way old thread,  but is there still not a way to replace vanilla textures? eg you still need to make a texture pack in other software to use with the custom mod?

I believe Mcreator only puts your new textures on newly created entities? Just importing textures with the correct vanilla names doesn't overwrite vanilla textures when the mod is loaded in game.

Way old thread,  but is there still not a way to replace vanilla textures? eg you still need to make a texture pack in other software to use with the custom mod?

You can make resource pack and add it to the resouces of the mod under domain minecraft and the resource pack will load with your mod.

Although this is not advised, changing textures is domain of resource packs.