I'm confused with this because if you reach agreement and a CAO ordered by consent then the proceedings will be concluded. There will be no ongoing proceedings to merge a SIO to; it would be a completely new application and the process will start again which is not something you want as proceedings can be lengthy and expensive. If the surname is important then you need to apply at the same time, just tick the box on the C100. The surname will likely go to final hearing as there isn't really anything to negotiate on, it is either changed or not. I'm not sure a surname change would be something you could use as leverage for agreement for a CAO as it has no relevance to safely caring for the child unless I'm not understanding you correctly.