If you did your changes via the interface as advised in the documentation, you won't loose anything. You can just install the new package over your current version.
Unfortunately, you can't just install it separately. Lots of things change between the different versions and even if you were to install the paypal pro plugin independently (which is possible), it would break when using it as it would use stuff of HikaShop so you don't have in your version.
You could also use another payment gateway...
For example, if you're in the USA, you could use authorize.net and their AIM API which behavior is like paypal pro but which was in HikaShop since a few months already.