I am not sure if you are seeing notifications as a result of the upgrade or if there are just now, new posts in a thread you have subscribed to.
There are a couple of ways to manage thread subscriptions.
The fast way to manage subscriptions is to go to the User CP link on the left side of the navigation bar (see the image below). Once you click the User CP, you'll see a Subscribed Thread option on the left part way down the page. Under that, click on List Subscriptions. From there, you can check the box to the right of the thread you no longer wish to subscribe to. Then at the both of the page on the right, there is a drop down selection menu. Use it to select Delete Subscription (see the image below). Then select the GO button.
//heartlandowners.org/attachments/subscriptions_cp.jpg
//heartlandowners.org/attachments/subscriptions_del.jpg
The other way to do mange (subscribe or unsubscribe) is to navigate to the thread in question. In your case, there is a thread link in the email notification you received. Click that link and your browser will take you to that thread. From the thread (see image below), on the Thread Menu bar, select Thread Tools. From the Thread Tools menu, select Subscribe to this Thread or in your case, you see Unsubscribe from this thread. Select as needed.
//heartlandowners.org/attachments/subscriptions.jpg