When you complete one of the renshuu-recommended schedules, you will get a message in your inbox congratulating you and suggesting which schedules to add next. Your previous schedules don't disappear when you add new ones, and it's up to you if or when to add the suggested next step (you could even add them before you finish the previous ones, if you felt like it).
Renshuu doesn't make you study anything automatically, or bar you from studying anything if you haven't finished the previous step. Everything is your choice of what, when, and how you study.
