I have an app I am offering for free for now. At some point, I want to be able to change it to a pay app.
The market doesn't allow a free app to become for-pay and doesn't allow for-pay apps to be priced at 0$.
So is it possible to just delete the original app and then reupload it as a for-pay app ? It's okay in my case if users aren't notified of updates.
Yes, it's possible. But your users won't be notified (which you've already said is ok). And (perhaps even bigger), you'll reset all ranking, reviews, and comments to nothing. I would really hate to lose all of that.
You might want to take a look at section 3.3 of the Android Market Developer Distribution Agreement.
Relevant bits:
You may also choose to distribute Products for free. You may not collect future charges from users for copies of the Products that those users were initially allowed to download for free. This is not intended to prevent distribution of free trial versions of the Product with an “upsell” option to obtain the full version of the Product: Such free trials for Products are encouraged. However, if you want to collect fees after the free trial expires, you must collect all fees for the full version of the Product through the Payment Processor on the Market.
Depending on how you interpret that, you may need to keep a free version available on the market, and have two versions: a free one, and a paid one with more features.
I suppose you could delete the free one then upload the paid one and call it a new "Product", but if you're at all unsure consider talking to a lawyer.
You can convert a paid app into a free app, but you can't convert a free app to a paid app. I discovered this when I tried to make a paid app free for beta testers, and then found I couldn't go back.
All you can do is delete the app from the market place, rename the package, and then upload the (now paid) app under the new name.
(This is why RpnCalcPro became RpnCalcProfessional)