Q: change country/region

so i can download apps not available in my country/region.

Sorry, but no.

The issue is that your credit or debit card credentials must be associated with the same country where you reside.

"Although you can browse the iTunes Store in any country without being signed in, you can only purchase content from the iTunes Store for your own country. This is enforced via the billing address associated with your credit card or other payment method that you use with the iTunes Store, rather than your actual geographic location."

From here >  The Complete Guide to Using the iTunes Store | iLounge Article

iTunes Store: Which types of items can I buy in my country?

You're welcome... 

What I should have said is that billing and country/region issues are the same ...

But the difference is that iTunes sells apps optimized for iOS devices such as iPad, iPhones, and the iPod touch, while the Mac App Store sells apps designed for the Mac OS X. Two different operating systems.

You can create a new Apple ID for the US and don't include any payment info, but obviously you will only be able to download the free apps

Are there any synching issues when using multiple accounts? eg, If I do this, when I download a free app from the 2nd store to my Mac, and then to my iPad/iPhone, the apps from the original store aren't erased? Can't think why this should happen...but pays to ask and be safe!

so do i need to log out my old ID and log in the new Apple ID for US to download the free apps?

Yes.

Similarly. when you want to update those US apps, you need to login to your US account.

No, there should be no issues. I have both Singapore and US apps/music on the same iPhone.

thank you so much