You will not be able to access all 9GB. You will only be able to see 5GB so if any of that other data matters to you, you should transfer it somewhere for safe keeping. You also won’t be able to backup or use many iCloud services or features such as iCloud email.
Once you downgrade, Apple does have the right to purge your data beyond 5GB.
Legal - iCloud - Apple
”Effects of cancellation
We will reduce your storage back to 5 GB and reimburse you no later than 14 days from the day on which we receive your cancellation notice. If you have used more than 5GB of storage during this period, you may not be able to create any more iCloud backups or use certain features until you have reduced your storage. We will use the same means of payment as you used for the transaction, and you will not incur any fees for such reimbursement.”