Should I update a popular blog post with a better call to action?

I have a blog post that is doing well (by my standards) and getting about 20-30 hits per day. It's been growing over time. I'm really excited about it.

My problem is that it's not generating as much traffic as I want to the main product page. I'm having a hard time really verifying, but I'm guessing about 3 visits per week (maybe less).

I plug my product at the end of the blog post with just a brief one sentence "you can do this same thing with our software in less than 1 minute" kind of copy. I want to improve that and experiment around with it.

First off, I don't want to mess with a good thing. I'm worried about changing the page and having google freak out and mess with my ranking for this post. Is this a real worry?

Second, any ideas about what kind of copy/content/buttons might make it more compelling?