I have owned both m60's and m80's. There are differences, which would be expected with any speaker company when comparing the top dog versus his little brother. The m80's are a bit more detailed which would be expected with dual tweeters/midrange woofers. Also, the m80's go a bit lower which would be expected with the larger cabinet, etc.
However, the m60's are great speakers that will pay as loud as most people can handle. The m80's are the top of the line and are capable of handling lots more power, however, most people will never have the power or roomsize to justify this volume.
A lot has to do with what receiver/amp your driving the speakers with, as to how the speaker will sound at higher volumes if the receiver is not clipping.
I would not say the m60's and 80's have an identical sound, but similar. They share the same drivers, etc. No the difference is not night and day, but there is a difference.
Yes, Axiom offers upgrades, your trade in value will depend on how long you've owned them.
