I guess the most obvious big jump was between Reason 5 & 6 - the latter integrated the features of the short-lived Record. Before 6, Reason didn't really have regular audio tracks so I guess 5 could be considered the last "old-school" version.
There are two versions that I know of, Reason and Reason Essentials (which is a cut down version) there was a version called Reason Adapted which was also bundled with products like hardware and ReCycle which I used to encourage people to buy and then upgrade from if savings were to be had....in other words instead of buying the full version of Reason then deciding to get ReCycle coupled with an upgradable version of Reason Adapted which would have saved them some cash if they had done things the other way around.
I don't know if they bundle Reason Essentials like they did with Reason Adapted or if you can do some kind of legacy upgrade by getting your hands on someones unused Reason Adapted license, I know I gave two of my unused licenses away to some kids so they could save some cash by upgrading to the full version instead of buying it outright but I don't know if they have closed that loophole or not, it's definitely worth looking into but you really need to do your homework.