How do I downgrade from Premier to Deluxe if my return is already in progress? I have no rental or investment income and Deluxe would meet my needs.

In TurboTax Online, you can clear and start over as long as you haven't paid or registered yet (free users are asked to register their accounts before they file).

Note: You can’t clear and start over using the mobile app. Instead, log in to your TurboTax account from a desktop or mobile browser to clear and start over.

1. Sign in to TurboTax Online and open your return by clicking the orange Take me to my return button.
2. With your screen maximized, click My Account (at the top of your screen toward the right) and select Clear & Start Over. (If on a tablet or phone, you may find the Clear & Start Over option by clicking the three parallel lines, or “hamburger”, in the top left corner.)
3. Answer Yes to Are you sure you want to clear your return?
4. After your entries have been cleared out, we'll prompt you to select a TurboTax product so you can start over from the very beginning.

A reason you might want to clear and start over is if you think you’re in a higher-priced version of TurboTax and don’t need to be. Have a look below under Related Information for more details.

If you're signed in under last year's login, once you clear and start over we'll automatically transfer last year's info over if you select the PLUS upgrade, Deluxe, Premier, or Self-Employed. Federal Free Edition (without PLUS) does not include the year-over-year transfer feature.

This is total bull**bleep**.  You have the ability to allow customers to downgrade, yet you don't because you're greedy bastards that have 0 ethics.  You know once someone has punched in all their info and hit upgrade they aren't going to want to restart their tax entry.  Such a shady business.  Wishing you bad luck on your pending lawsuits Intuit Scamtax.

For those accessing this page, use the links supplied not the no longer applicable procedure in this answer.

I was in the same predicament, however I found a loop hole!

Simply go to tools --> Delete a Tax Document --> and start deleting all your tax documents. 

Voila! Your numbers for both your Fed and State will eventually roll back to 0, and you can begin entering your information like you initally did!