How to use an existing Apple ID to download free apps from App Store without using Credit Card?

As I commented - try setting your payment type to None in iTunes

Open iTunes, and go to the iTunes Store, then click on your account on the top right. It'll take you to your account page where you can change your type.

For those still having this issue (like I did) Select the "Redeem a code or gift" option then sign in again. Once it gives the line to input the code, the option to select "None" shows up.

I just had the same problem: an existing Apple ID I made couldn't be used for downloading free apps in the App Store without entering credit card information. But here's the steps I used to free up the old account and make a new one:

1. Go to appleid.apple.com and sign in using the existing ID.
2. Update your email address to another email address of yours. This will also change your Apple ID to that email address.
3. Verify the account using the email.
4. In iTunes, try to download a free app, and when it asks you to sign in, choose the Create Apple ID button.
5. Create a new Apple ID using the old, original email you removed from your Apple ID in step 2.
6. Choose none while entering other basic info (name, address, etc) and voila, you can download free apps using the same Apple ID.

But if you had used that first Apple ID to make purchases on your iPod shuffle, they wont appear.

I just use this trick because I wanted to download free apps using that particular email address as my Apple ID (while not entering my billing information).

1. Go into iTunes and make sure you are NOT SIGNED IN.
2. Go to the App Store and click "Redeem"
3. When it asks you to review your account details, just click the "Review" button
4. After agreeing to the terms, there should be a "None" button under payment details!