You can keep the same sales page and buy button on your page if you want to. You will then want to create a copy of that product, so you can create your other subscription rate (each price break needs it's own product).
Once you have the second one created, you can add it to the front end of your existing funnel. You will want to sync your two front end products to ensure that all previously approved affiliates are approved for them both.
In your product editor, you will want to check the 'Subscription or Multiple Payment' box. That will then open the subscription settings box for you. In that box, you will want to put how often the customer is charged, for how long, and what commission you want to payout on the first payment.
Under that is a section you can use to set up a trial period. You can see exactly where you need to set this up here: http://screencast.com/t/WSBgZrRE
Changing the product to a subscription will not affect previous purchases.