No. JOIN Plan does not replace the JOIN app. They serve different purposes and work best alongside each other.
The JOIN app is where you manage your training. You set your availability, adjust your goal, submit RPE feedback, and interact with your plan day to day. All changes to your training plan are made in the app.
JOIN Plan is a web-based view only. It gives you a broader picture of your training, covering your fitness history, power records, calendar, and activity detail, but you cannot make any changes from it.
Think of JOIN Plan as the dashboard for your training data, and the JOIN app as where the training actually happens.