You might want to read this Support Article: https://www.quicken.com/support/how-do-i-manage-my-quicken-subscription
Actually, you can downgrade now. Contact Q Support, via the link at the top of this page and tell them what you want to do.
The file layouts are identical, it's just that some features aren't turned on in the lower versions. All of your data will be intact … but the Rental functions will be unavailable
DON'T go to Starter as it doesn't support investment accts.
You might even get a refund or an extension of your subscription by doing this.