Rewards are cumulative! Glorious Leader is very generous and has allowed Comrades to earn all rewards up to the Rank they completed.
So, if you reach Rank 10 when the Event ends, you will receive all Rewards for Ranks 1-9, but not Rank 10. You must finish all missions for each Rank to be eligible for rewards!
Remember: Rank rewards will be claimed at the end of the Event.