That's the option I took. Except I took the option during a half term break and spent the break transferring all the viable content from 1.9 to 2.4. It took 5 days but it worked. I restored the 1.9 backups into 2.4 for the majority of it but some of the courses for Modern Foreign Languages (MFL) used the Poodll v1 flashcard activity that doesn't transfer so I rebuilt the flashcards in the improved Poodll v2. No one has objected thus far, I've cleaned out students who have left/haven't logged in for anything up to 4 years and reset everyone...
Next task is really staff training.