@JillStiles ...
No, you won't lose all your assignments. When you have a brand new course shell in Canvas, you can import (copy) content from the existing course into your new course. All your assignments (and any student submissions) stay in the original course. For your new course, you'll have a different set of students and all your content in that course so students can submit work to you there. You'll still have access to your past course, too.
In general, this is a pretty common setup for lots of schools who use Canvas. Rather than using one single course from semester to semester, you can have separate sections of the same course from semester to semester to keep your students separated. This also help keeping your assignments/grading organized from semester to semester. You never know...you might have certain assignments in your original course, but then for future versions of the course, your assignments might have slight changes that need to be made.
Anyway...I hope this helps to explain things a bit. Sing out if you have any questions...thanks!