No, you won't have to pay again if you're using the same Microsoft account as before. It'll still say 'buy' in the store, but when you click on the 'buy' button, you'll get a message saying you've bought this before and do you want to reinstall.
You won't get billed again. I've downloaded a few paid apps I've bought on WP7 and I've not been refilled for downloading on my new WP8 phone.
I have a similar question. I have a computer running windows 8 and I just bought a windows phone, the nokia lumia 1520. I cant seem to figure out how to sync the apps I purchased on my computer with my phone and vice versa. It make me rebuy apps when I download them on my phone. Is there any way to sync the apps?
