What specific tactics do you use to test a feature set that's planned for an upgrade of a current product?
Existing user survey
When you are a developer, a must read would be the development philosophies of 37Signals. They by far have the best book on development. They tell you what you need to do and what you dont.
You are developing a feature. First of all, did you think of the feature or was it recommended to you? you need to keep your app as uncluttered as you can. If your competitor has 100 features, you don't really need to have 101 features. You can make do with 50 features that work more efficiently than your competitors.
Moving on to your current question..
If you read the book called Purple Cow (great read) you would see that each product/niche has a curve (which was introduced in the book called crossing the chasm) You can look at it here. http://news.cnet.com/i/tim//2010/05/17/page19.gif You need to start filling your customer base from left to right and you can only get to the juicy middle part if you complete the ones on left.
The ones on the left are the people who are called the Sneezers who will spread your IDEA VIRUS to those juicy potential customers. Only they can make your product more valuable. Those people are Innovators and Early Adopters they are the ones who want first of everything. SO MAKE THOSE PEOPLE TEST YOUR PRODUCT/Feature.
Getting back to 37Signals philosophy, you need to do small testing. You need to find a group of people who are enthusiastic about your product and want to help you (Linking to Crossing the Chasm, the SNEEZERS) and have them test the product. Small scale testing is always better. Find 20-30 sneezers let them test it for you. They will tell you what to improve upon and if they like what you got, they will spread your "virus" (ideavirus that is!)
That was a lot of text for just one small answer but I wanted to tell you why you should test with Sneezers. Along the way I've mentioned some great books that I have read and loved. I suggest you read them (mainly Getting Real, Rework and The Purple Cow)
P.S: Avoid surveys they are boring and wont give you a very accurate information to help you. What are you going to ask them? Do you want to pay us more? Of course not! Just do the testing on the sneezers, if they like your product they will spread the word and everyone will buy the upgrade.
Take care, Bhargav.