Hi @RebeccaCS , unfortunately there is no native support by Asana’s Rules to trigger based on the status of a project (being completed). Rules in Asana will only act upon the task that is being triggered.
I’m not sure how a project could be marked ‘complete’ if there are still ‘incomplete’ tasks remaining… 
but for the time being, the best would be to review all incomplete tasks (set a filter) and then multi-home them into the next sprint by adding them (instead of moving them) so you don’t loose insight of what was actually not completed in the previous sprints.
For multihoming several tasks at once, you could follow this quick guide: