I recommend starting with a schematic design of all the changes you're planning. Then consider how many phases to divide the entire project into. Ideally the work can be broken up so that you're not having to un-do work that was previously done.
Once the phases have been determined, you can move ahead with construction drawings for the first phase. The construction drawings are the more detailed drawings that will be used to get your building permit and that the contractors will use to construct the project.
You could have construction drawings done for all of the remaining phases of the project right away, but I think that it usually makes sense to wait until you're closer to the time when those additional phases will be constructed. You may find that your thoughts about what you need change over time or as you live with the results of the first phase of work. Additionally, the building codes change over time, and construction drawings done now may need to be changed down the road to reflect the code in effect at the time of construction.