Can I change the thumbnail image associated with a .mov movie?

It seems like in earlier versions of QuickTime there was an option to set a frame as the thumbnail image.

Are you referring to the "poster frame" or the Finder "icon" image? The former can be set in QT 7 Pro (or iTunes) while the latter can be changed by dropping a replacement image to Finder "Info" window.

I have Quick Time Pro, but don't see this option in any of the menus.

"View > Set Poster Frame" menu option in the QT Pro Player. The selected frame will then be used as the poster frame in apps like iTunes or iWeb. This is as opposed to changing the Finder icon image by dropping an appropriate new image to the icon in the Finder "Info" window.

What is the best way to tell,

The "About" window contains the QT symbol with the word "Pro" superimposed on it, The Registration window says QT 7 Pro" below the key, you can export to other compression formats, you can acces the properties window, etc.

and how do I upgrade?

You purchase and enter a QT 7 Pro key from the Apple store for $29.99 to unlock the QT 7 Player's "Pro" features.

Also, is there an easier way of dropping in a new image to the movie's "Get info" besides taking a screenshot of the exact frame I want?

Depends on what you consider "easier" and whether or not you want the frame taken from the file or somewhere else. With QT Pro you can simply select the exact frame desired and "export" it as a "Picture" for dropping in the "Info" Window. Sometime I create a custom frame in Photoshop using a background and title of my own making or simply "trim" a teatrical poster, DVD, or video cassette image to use as a custom Finder icon image. The basic question is, "How much work do you want to invest and how good do you want it to look?" The choices are entirely up to you.

FYI: If you don't have QT 7 Pro, you can gain many (but not all) of the "Pro" features using the free MPEG Streamclip application available on the Internet.

Jon, your post is time-stamped June 10, 2010, and your sig says your are using Mac OS 10.6, yet your answer seems to come from three or four years ago. QuickTime Pro 7? We're on QuickTime 10 now, and the "Pro" option (along with a lot of convenient Pro features) seem to be gone now. So I suppose the problem for those of us living in June 2010 goes unanswered.

Snow Leopard users can have (and use) two versions of QuickTime (X and 7.6.6).
Only 7.6.6 can be upgraded to "Pro". QuickTime X has no user defined settings and only a handful of export options.
You don't need the Pro upgrade to make a Finder icon "preview". These also work in Front Row and iTunes.
Move the playhead of QuickTime Player 7.6.6 to the frame you wish to use as the Finder icon.
Edit menu / Copy
This places the video frame onto the Mac's "clipboard" ready for pasting.
Do a "Get Info" on the QuickTime file and you'll see a small icon in the upper left corner.
Single click on it and it will highlight (outline). Command-v to Paste the image on the clipboard to the QuickTime file.

Jon, your post is time-stamped June 10, 2010, and your sig says your are using Mac OS 10.6, yet your answer seems to come from three or four years ago. QuickTime Pro 7?

Sorry. On a bit of a vacation. For minor editing and routines daily work routines, I still use QT 7 Pro as my primary general utility. As QT Kirk indicated, most "power" users tend to rely on QT 7 Pro as the more powerful utility which, other than resorting to full Pro video editing applications, still is the best solution for quick layering, blending, and/or masking operations in QT files. Remember, The QT X player is still in its initial release and will likely be under further development for years to come.

So I suppose the problem for those of us living in June 2010 goes unanswered.

In addition to the apps and work flow described by QT Kirk above, IWeb, the "Export for Web" option, as well as "Movie to Picture", MPEG Streamclip "Export Frame", or "Window" screen capture options all provide methods of creating/saving non-temporary "Poster" content for both internal and external QT file use. In short, there are many solutions and you can litereally spend hours or days trying to decide which which solution best fits your particular work flow depending on whether you need to keep/save the source image and/or modify/enhance/title it before use.

You install QuickTime Player 7.6.3 from the Snow Leopard DVD.
Once installed the 10.6.3 OS update moves it to 7.6.6.