There are many, many changes. Kirby makes a good, simple and reliable vacuum. Even if a Dyson lasts 10 yearsI will list some by generation.
Dual sanitronic 50: two speed switch.
Dual sanitronic: many changes, Kirby states there was 26. Main one being the new handle.
Classic: launched the "second generation" with new switch position, updated sani em-tor and a wider nozzle.
Classic omega: updated toe touch control.
Classic III: visual belt window, triple cushion vibration head, updated shag king, introduced the jigsaw.
Tradition: introduced disposable bags.
Heritage: new attachments, improved bag system.
Heritage 84: redesign for their 70th anniversary.
Heritage II: mini em-tor replaced sani-emtor, power indicator added, new toe-touch control, micro matic added.
Heritage II (1985): new nozzle, new attachment caddy, zipbrush added as an optional extra.
Heritage II (1986) : brush roll performance indicator added, extended warranty.
Heritage II legend: updated brush roll, updated bag system.
Legend II: ball bearing brush roll introduced
G3: over 60 changes made, most notably introduced techdrive.
G4: updated handle tilt latch, micron magic bags introduced.
G5: updated styling, pull out belt lifter handle, updated fan case gasket, updated fan.
G6: HEPA bags added.
G7: updated brushroll, updated fan, updated bag zipper.
G7 diamond: updated bag zipper, two speed switch.
Sentria: back saver handle, led headlights, higher nozzle bumper, updated upholstery tool.
Sentria 2: updated mini-emtor, f style bags introduced, updated zip brush.
Avalir: added handle, updated shampooer.
avalir platinum: noise reduction stuff.
As you can see there has been a lot of changes so when people go about complaining they are the same vacuum it really grinds my gears. I think I don't have anything wrong, but if I do please tell me.