The only visible, non-aesthetic changes you’ll be able to see are the vents on the newest black model, an Active/Passive switch on the input (in place of dual input jacks), and an FX Loop Bypass switch on the back.
@agedhorse has mentioned that there have been some internal changes over the years, but there are none that I am aware of that are necessary for a killer functioning amp. There are still people rocking the first generation Walkabouts without issues (though I would probably at least get the two switch mods done to one of those early ones because signal cutouts from oxidation of unused jacks are a known issue in the earliest Walkabouts without those switches).
The vented panel is probably a great option if you find yourself slamming your amp in hot rooms a lot, but I’ve done that with my non-vented Walkabout many times with no issues.