Hi,
1. It's a one-off fee. After that you can use your HikaShop Business for as long as you want.
On top of that, you get a 1 year free subscription to updates and support that you can renew for 60€ a year, but it's not mandatory. Your HikaShop will continue to run on your website after that even if you don't renew. However, you won't be able to update it and thus, if you update your Joomla, you might end up at some point with bugs due to things changing in Joomla that would require also updating your HikaShop.
2. yes, you can just upgrade. You will have to install the HikaShop Business install package and it will automaticale perform the upgrade. You won't loose your data, like a normal update of HikaShop.