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. 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.