Not sure if upgrade is the right word. I still have my O1. I was going to send it in for trade in, but haven't gotten around to it yet and might not at all. So far I still have all the kindles I bought but one. I gave the PW1 to my husband as I hated it. But everything else, is still mine and registered and functional. The K1 is getting a bit flakey now, but it is 10 years old after all.
I still grab the Voyage and the Oasis at times, depending on where I am at in my living space. I do read pretty much on my O2 now only. The others feel so tiny now.
For me main reason I got the O2 was to continue to have a kindle with actual buttons. My O1, its device battery is getting a bit worn. Used to last 7 hours out of the cover, now its more like 4 or so. So I just wanted to make sure I have another button device. I get really nervous each time they have new kindles as I am afraid to not have an option anymore.