Replaces an existing block with new content. This function will replace a block of the template and all blocks contained in it and add a new block instead. This means you can dynamically change your template.
Sigma analyses the way you've nested blocks and knows which block belongs into another block. This nesting information helps to make the API short and simple. Replacing blocks does not only mean that Sigma has to update the nesting information (relatively time consuming task) but you have to make sure that you do not get confused due to the template change yourself.