Star Wars d6 2nd edition - came out 7 years after 1st edition and compiled a lot of rule tweaks into the core book. Also, the changes were not to stats but in how the system worked, so tons of supplementary material were not invalidated by this update - GOOD
SWd6 2nd ed revised and expanded - only 2 years after 2nd edition (that was bad) but not so much a rules update as a core book update that was just incredible - full color, better organization, much better art and again it really didn;t invalidate anything that had come before, other than your core rulebooks - GOOD
Shadowrun 2nd ed - Cleaned up a lot of rules issues discovered in the 3-year run of SR1, set the foundation for a nice long run, and didn't invalidate 1st ed material - GOOD
Shadowrun 3rd ed - 6 years after 2nd - More rules tweaks and included conversion information in the back for almost all of the 2nd edition supplements which was awesome at the time, set the stage for another 6 year run - GOOD
MegaTraveller - 10 years after Traveller first appeared it came back in a new boxed set with 3 books that included greatly expanded character generation, ship and vehicle construction, and setting detail. It was "more" Traveller in every way and was excellent for both old and new players. The rules were an evolution of the prior set, not a total change which also left most of the prior material useable as written. Now as we found out later it had a LOT of errors and in the pre-internet era it was a lot harder to track down the errata sheets, and it never was as well supported as the black book edition but at the time it was great - GOOD
Traveller New Era - 5 years after Mega, we got the 3rd edition of Traveller and it was a radical change mechanically, rendering most of the older material unusable mechanically. I found the mechanics clean, logical, and to some degree realistic, but also kind of plain and boring. It also included a major timeline advance taking it from a classic galactic empire type setting to a post-apocalyptic in space setting, which while a cool idea, did not go over well as the mainline Traveller setting with existing fans. I gave it a try and liked it to some degree, but effectively it had nothing to do with the prior Traveller game either mechanically, thematically, or setting-wise and I'm going to call it the textbook example of how not to do an edition change - BAD
Traveller 4th - less than 5 years after TNE we got a whole slew of black-covered books from a previously unknown company that went retro in both look and setting and kind of mechanically as well. The setting was roled back to he start of the 3rd Imperium, the main Traveller setting for 1st and Mega editions - not a bad move. Mechanically it went back to d6's for task resolution but the system was quite a bit different, so a partial win there, but it was not compatible with the older material so in the end it's still a break. The game was also horrendously full of typo's and errors and died a quick death - BAD
Mongoose Traveller - roughly 10 years after the last edition Mongoose came out with a new edition intended to be a more generic set of rules that could be used for a variety of science fiction campaigns much like the original 3-book set of Traveller. It went back to a system that was very similar to original Traveller for task resolution, had a good basic starship construction and combat system that was again similar to the old, and had a look that was similar as well. It was a huge win being both appealing to old-school Traveller players and relatively rules-light which had been the trend in a lot of games over the last few years. Setting material has been published in separate books covering the classic Third Imperium well and adding settings as diverse as Hammer's Slammers and Babylon 5. In my opinion this is a textbook example of how to put out a new edition of a game that has a large fanbase but has been dormant for a while - GOOD
Twilight 2000 2nd edition - 6 years after 1st edition GDW rolled out a brand new system that completely invalidated the mechanical elements of the very well-supported T2K 1st edition. Mechanically this system was very limiting and T2K "Version 2.2" came out within 2 years which improved the system greatly, but 1st edition was very dry, very plain, with no "chrome" to make things interesting. Setting wise the game's background was altered a bit, but the core concept was the same so it avoided the 2nd major misstep of TNE noted above. It also largely reused artwork from the 1st edition material to help keep the feel of the game as T2K had a lot of very nice detailed pencil artwork that added a lot to the look and feel of the game and it was a smart move to keep this. However, considering they had to redo the rulebook within 2 years I'm going to call this one too a fail - BAD
Twilight 2012 - I admit I haven't read this one but I know it exists and I've read some reviews. Let's see, different mechanics that have little to nothing to do with either of the previous editions, a totally different background though technically it is the same - post nuclear America/Europe, plus it's a mechanically heavy game in an era of lighter rules so that's not looking good. On the plus side, it's been close to 20 years since the last edition was released so it has that going for it...really more pointless than BAD - it should have been a GURPS setting book for the realists and a Savage Worlds Book for the less hardcore among us.
Midnight 2nd edition and Conan 2nd edition - these were alternate d20 fantasy games that came out in the early 2000's and then turned around and put out a 2nd edition within 2 or 3 years and then republished most of the material already out. I'm not saying they weren't needed, but they felt like a cash grab coming that quickly considering how little changed mechanically and I dumped both games when it happened - BAD.
Arcana Evolved - coming out about 3 years after Arcana Unearthed this was to at least some degree driven by WOTC publishing a very similarly named D&D supplement and a desire to avoid confusion. It added in material from 2 prior supplements (which were not republished), added new races, new classes, expanded the magic system, and raised the level cap. It also went to full color for the entire book and spawned several very nice supplements that were not reprints. Despite the short turnaround time this was a vast improvement on the already very good original and didn't require the repurchase of the prior supplements, mitigating some of the usual edition change pain. - GOOD
Gamma World 2nd edition- kept the same system as 1st edition, added a lot of material, and gave it a great look that was still being used in some ways over 10 years later - Good
Gamma World 3rd edition - total system revamp, some background/setting changes, lots of errors and errata - I liked it for what it was, but it was a big change from 1 & 2 and drove off some fans. In a lot of ways it seemed like change for the sake of change - BAD
Gamma World 4th edition - another system change but it went back to something at least similar to 1st and 2nd in some ways. It added classes and a more systematic approach in many ways. You can see a lot of precursors to d20 in this version and it played very well. It also had been 8 years since 3rd. - GOOD
That's probably enough for now. One thing to note, I don't typically describe new editions as "upgrades" - that's because while it's always a change, it's not always better and upgrade implies improvement. it also implies an improvement to an existing base and in some of the more radical changes, (TNE, D&D4) there is no mechanical connection to the prior system so it can hardly be called an upgrade. Switching to an entirely different system is more properly termed a migration or a switchover than an upgrade.