Hi Jabramo,
As you’ve already noted an exchange under service plan is done by feature set. Essentially the store will replace your TV by matching or bettering your existing features. When carrying out service plan exchanges the price of the product has no bearing except to say that the exchange model can’t exceed the original purchase price of the covered product. It’s a natural occurrence to see the value of an exchange model be less than the original purchase price of any covered product as market trends demand that as new feature sets are added to products the value of old ones decreases. In terms of will you get a dynex or a samsung, typically the stores will try to align you to the closest matching product within the same brand.
Exceptions can and do happen where no other options are available but as a
whole the stores will try to work with customers to make the experience of an
exchange a fun one people can be happy with.
Once the store has established a valid exchange product with you (by
features), you can take the retail value of that exchange product and have it
applied to any item you wanted to purchase, so yes this is a great time for an
upgrade. 
Hope that helps!
