Hi,
What old and new interfaces are you talking about ?
There is only one interface for the PayPal Standard API. The HikaShop PayPal plugin implements that API, and thus you get the interface of that API.
If you want another interface of PayPal, then it requires using another API, and thus another payment plugin.
For example, if you want the PayPal advanced API interface, you need to use the PayPal Advanced payment plugin.
So as I said in my previous message here, first check with the PayPal support which API is necessary for the interface you want (they have more than 2 of them, we have 6 payment plugins for PayPal, and still don't cover all their APIs) and then you can see if there is already a payment plugin or not implementing that API in order to get that interface.