Yes, you can plan for future weeks.
To begin, go to Clients and select the client whose plan you want to amend. Click the Nutrition tab.
Select the week in advance to amend the plan for in the date dropdown
Click Edit Week
It will use your latest meal plan, so edit it to fit your clients needs
Remember to Publish Changes once you are happy. If you don’t publish changes immediately, you will not lose any edits, but they will be in draft state.