In-app purchases of an app downloaded from a different country

A few months ago, I purchased a number of applications from iTunes US using a US-based credit card even though I'm not from the USA; from the PH, actually. (I used AmEx Virtual Pay offered by my local telco provider)

Now, I recently moved to Japan and upon seeing these iTunes cards on the convenience stores, I am really tempted to buy some. (I don't have a JP-based credit card and I cannot, should not, must not use my PH-based CC to do purchases).

Which brings me now to my problem, I want to do some in-app purchases on the apps I purchased on the iTunes US using (possibly) the iTunes card JP. Is this possible? Will changing just the country of my iTunes work? Since I'm thinking that even though the app was downloaded from a different country, after all, I'm still using the same account.

I understand that if I decide to download new paid apps, I could just change my country to Japan and use the iTunes card, but will this work with in-app purchases?

Let me know if my question's unclear. Thanks.