First of all don't confuse Freeview with Freesat From Sky (***). Freeview is Digital Terestrial (aerial) based and *** is Satellite (Dish) based and they have a different programme line up.
If you have Sky+ you proably have a quad LNB - the thing that your cables are attached to at the dish.
So, to use your old Sky box upstairs run a cable from the dish to it. (You should find a spare connector at the dish.) If Sky give you a new card for your new box, your old one will work in your old box as *** card. If not your old box will still be able to receive BBC, ITV and lots more including Film 4. If you want Ch4, Ch5 and Sky Three, you'll need a *** card at £20 - from Sky. There are a huge niumber of post son this, why not try a search.