@NanetteBroderic The free version uses the address canvas.instructure.com and the paid version will use yourschoolname.instructure.com. They are completely different so students/teachers can be on both (I have accounts with about 5 different Canvas systems). Students would need to know which system has what information so I would recommend having a cut off date and then everyone move to the paid version which could include users that previously were on the free version.
If you have both running and some using one vs the other, your students will very likely be confused and could use this as an excuse to not do the work. When we switched from Moodle to Canvas we unfortunately had to run both for a period of time and students complained about being very confused and not knowing which instructors were using which system which did result in missed assignments.
Hope this helps answer your question!
-Nick