Change the price of in-app purchase item after publishing android

Yes you can change prices at will.

You change the prices on the Developer console not in the application, of course if you have graphics/text in the app that shows prices you need to figure out how to update those, maybe pull them from a server? See: Billing Catalog and Billing Testing

Yes, you can change prices of managed and unmanaged in-app products, but NOT for subscriptions. This is because users will always see the latest price of products as they purchase them, so changing the price of a managed/unmanaged product is fine. However, customers who have already purchased a subscription and agreed to a certain price per month (or per year) cannot be made to pay a different price without knowing it.