One thing, you should check first is if HP provides you with all the drivers for Windows7 for your laptop. If they promise easy upgrade, then most likely they do, but check to be sure.

HP Support & Drivers

As for the upgrade being free (or cheaper). You should either have documentation about this received inside the box your computer came in. Or alternatively through an easy upgrade solution software installed on your Laptop (look through programs installed). If you can't find either of those, then check with the store, where you bought it.