Try deleting the cache associated with the App Store...
Quit the App Store if it's open.
Open the Finder. From the Finder menu bar click Go > Go to Folder
Type or copy / paste: ~/Library/Caches/com.apple.appstore
Click Go then move the Cache.db file from the com.apple.appstore folder to the Trash.
Relaunch the App Store.
See if you can edit your payment credentials now.
Billing policy is the same for both the iTunes and Mac App Stores.
Before I tried your suggestion, I ran into new problems:
1. I logged in to the iMac with a brand new user account. I thought it was going to work, verified and then got an error message "Unable to complete your purchase. Please contact iTunes support."
2. Rebooted. Logged in to the iMac with my account. Tried again and got error 100. Try again later.
I just tried your suggestion. The app store started faster, but I still get error 100 try again later. I never get to the verify step.
Thanks for the help.
Okay, the store is messed up.
I decided to go through the steps and change the credit card to NONE in both iTunes account and App Store account. I changed them both to none and saved.
I exited both. Re-entered, called up the account and both were NONE.
I re-entered a completely different credit card and all the info. I clicked Done. I re-entered my billing info into both. Clicked Done. I exited each. I restarted each and checked the account. Both had the new info.
I went to the App I want to buy and tried to buy it. Got error 100, try again later/contact iTunes support.
I checked the account info in both. BOTH WERE SWITCHED BACK TO THE ORIGINAL CREDIT CARD.
This has gone from an annoyance to a security issue with me now. Obviously they are keeping my old credit card info on file somewhere. That's not right.
I am having the same trouble! I keep getting told my security code for my cc is invalid, IT'S NOT! I have tried over and over and it still says the same thing...so I tried 2 other cards and still same thing! Then when I went to log back in it still had the ORIGINAL card that I put on there! What's going on?! It was working fine before and it validated $1.00 on my original card on 8/18 (it still shows the charge on my pending account charges through my card)!