In particular, most of the games/apps I've tried that use that "upgrade from free via in-app purchase" model make it annoyingly easy to upgrade. They remind you when you finish a level. They remind you when you start the app, and when you go back to the main menu. They have buttons for full-version features that just pop up a "Sorry, you need to upgrade to do that, click here to do so!" dialog. They beat you over the head with the idea of upgrading until you submit, and going by the sales figures, it seems to work. That's how they make conversions, and if you're going to leave that out you're probably better off pursuing a different style of monetization.