No, unless you reschedule, you keep what you got once those receivers are activated.
Keep in mind that installers have very little control over what they get. When Dish is out of a receiver type, they could be out of it for a month or more, or only get in a few that go out fast. As a sub, if receivers are running low, I either take an "equivalent replacement" or I don't get any work. It's the same with new vs. refurbs; you get whatever they give you.
With HD, you have a good chance of getting a new, recent-model receiver. With SD, chances are it's a refurb, as Dish has nearly elmininated the manufacture of SD receivers, and is using the remans pulled from HD upgrades and Eastern Arc areas to install new SD jobs. It's been many months since I've seen a new 625, 322, or 311.