If you change a student's level, would they lose their previous data or need to start the strategies again? E.g., would their Benchmark chart suddenly look empty?

Not to worry, your student would not lose any data from the current level when you reassign. Your students would still be able to access their previous reading level and their progress chart from their dashboard. As a teacher, you would still be able to access your student’s previous reading levels from their individual profile pages. If time permits you are able to allow your students to work on their Pretests. Otherwise, they can continue where they have left off. e.g., Benchmarks or Post Tests.