Windows – I keep being asked by iTunes for password and credit card verification, and getting warning emails

According to Apple's page, it uses the following order for song purchases:

• Song credits.
• Gift certificate, prepaid card, or allowance credits.
• Credit cards.

Thus, it would appear that you cannot charge a credit card without using up your credits or gift certificates first.

However, similar to what @Nick said, you could just open up another account, add the credit card to it, and then gift the items you want to your original account.

Update: Based on my own tests of self-gifting, these are my findings:

• Gifts will deduct money from the credit card on your account; they will not use up song credits nor gift card credit. This means that you do not need to open another account, just add it as a credit card and gift it to yourself.
• You cannot send a gift to the same email address as your iTunes account. This can easily be circumvented by sending it to one of your other email accounts.
• The easiest method of gifting to yourself is to select the print option. It will show you a code which you then use to redeem the song. It will allow you to redeem the code on the same account.

So... unfortunately the answer is...

Yes, but there are ways around it...

All the apps you bought in country A are permanently assigned to your AppleID. Once you move to country B and you updated your details (billing address) you:

• still can use them
• you cannot re-download them
• you cannot update them
• you cannot convert your music to iTunes Plus (but still can listen, obviously)

There are two solutions to this:

1. Switch Store's country and update your billing details each time you want to get your apps up to date (or re-download them). But this is a rather painful process, so a better solution is to...
2. Create a new AppleID with the billing details for the country B. Switching between AppleIDs is much easier and quicker than switching Store's country and updating your billing address.

Let me quote why it has to be this way:

[...] this how iTunes has been designed.

So. It's gonna be problematic, but nothing can be done really. :(