Will data be lost if I import an updated csv (say, with late-joining students) into an existing organization?

Everything will be fine: High5.ID works as you hope it will.

Scenario:

• Import students.csv
  • Get 1000 records -> 1000 students created -> 1000 inds in DB
• Import teachers.csv
  • Get 200 records -> 200 teachers created -> 1200 inds in DB
• Edit one row of students.csv to modify a kid’s name
• Import students.csv
  • Get 1000 records, but only 1 update -> 1 student updated -> 1200 inds in DB
• Add one row to teachers.csv to add a new teacher
• Import teachers.csv
  • Get 200 records, but only 1 new entry -> 1 teacher added -> 1201 inds in DB

In sum, “existing data will only be affected if the CSV  you are importing changes the value of a column for a given individual”