If a class needs longer, push the end date out. Work already submitted is not affected.
Changing the date
Open My Library from the sidebar and click your pathway. Set the type filter to Pathways if you have a lot of content.
Click Assign, find the classroom's row and change its End Date. Each classroom is set separately, so you can extend one class without touching the others.
Students who were locked out by the old date can get back in as soon as you save.