I have a very simple question:
Is it possible to change the price of an InApp product after the application is Approved?
It is a very simple/stupid question, but
Yes it is, but make sure you don't hard code the price. Any label showing the price, pull the price from the products and localise the currency before adding to a label. This way it'll always show the correct price in your app.
Yes, it is. You can change the price at any time in iTunes Connect.
Go to iTunes Connect -> Manage Your Applications
Select your application from the list.
To change application price click Rights and Pricing and you're there.
For In-App purchases prices you go to Manage In-App purchases, click on the line of the InApp you're interested in and again you'te there.