Before HikaShop redirects the user to paypal, it has to create the order. Because when paypal sends the payment notification, HikaShop needs to be able to identify the payment notification with the order of the user. The only way to do that is to precreate the order before the user goes to paypal.
Thus, it's not possible to change the payment method afterwards.
When the user returns on the website HikaShop doesn't know the status of the payment. Plus, the user could just not come back to the website after the payment.
If you do the modification I wrote that will keep the cart of the user. Then, you could some similar code in the view after_end which is used when the user comes back (unless you specify a return URL) so that the cart would be flushed at that time.
But, then, if the user pays by check, the cart wouldn't be flushed at the end of the order. So instead of removing the code of the previous post, you should add a check to do it only based on the payment method.
But then, the problem is the same: if the user changes his mind and want to pay by paypal after finishing his checkout and selected check, that's not possible either...