RT (5 of 5): Seriously though, people who try to beat their beliefs and theories into people as facts can have negative results, and not only for this topic but many other topics in the Transformers lore (FIBRIR/FIRRIB, reproduction, etc). Will we ever get official answers from Hasbro? Who knows. Some of these topics have been answered with facts yet there are still many out there that will attest them. Least one can do at this point is make sure to point out when these accusations are wrong with the proper knowledge they are instilled with, at the very least in hopes that others will ignore the "raving derelicts" and listen to common sense.
RT (4 of 5): For all we know Magnus may have just been fighting Decepticons back on Cybertron, or even in another galaxy, we just never heard of him till Scramble City (his first official appearance). Either way it's difficult to say whether or not Dion became Ultra Magnus or anyone else for that matter. Dion's fate seems to be undetermined. Now should you believe the TFCC fiction fills in the blanks it will say that Dion was just repaired and joined the elite guard under Metalhawk's command, and Ultra Magnus was Magnum before becoming "Ultra". But at the end of the day there's really nothing official that tells us what happened to Dion, and where/who Ultra Magnus, Ironhide, Kup, and Huffer was/were before arriving to Earth.
By all means, fill these blanks in yourself, sometimes we have to make ourselves happier with our huge multiverse'd franchise.
RT (3 of 5): Perhaps Magnus came to Earth and earned Optimus's respect and admiration shortly after season 2 ended. The problem is there is 2 huge gray areas in the Transformers fiction. One before the Autobots left Cybertron (attempted to be filled in with War Within, and the upcoming War for Cybertron), and the period between season 2 and the 1986 movie.
Between the second season of Transformers and the 86 movie (time frame 1985-2005) there was a 20 year gap, enough time for even Spike to get married and start a family. In this time it's safe to assume this is when the new Autobots from the 86 movie showed up. 20 years is plenty of time for Ultra Magnus to show up and impress Optimus, probably why Ultra Magnus was put in charge of Mertroplex's constuction. But what was he doing/who was he before coming to Earth? Who knows? But we can't say for sure he was Dion.
RT (2 of 5): Let's take for instance Dion, who seems to be in recent limelight on our boards. Now there are multiple theories on who he becomes, Ironhide, Kup, Huffer, and most predominant, Ultra Magnus. But why is there such a large amount of people that believe this is the case? Could Dion not have just ceased to function? Could he not have just been repaired and remained Dion, never to be heard from again? Who knows if he was even rebuilt into any of those 4 previously mentioned? There are numerous possibilities...so why is "he was rebuilt into Ultra Magnus" the number one considered by fans?
I suppose the most obvious answer would be because he was the successor of the matrix and leadership after Optimus died....he did just come out of nowhere and was referred to as an "old friend", but how does that suggest he was Dion? Perhaps Optimus and Ultra Magnus fought together between Optimus's creation to his team leaving Cybertron.
Random thought (1 of 5): Am I really that crazy for trying to differentiate fact from fiction? I mean I'm cool with people having their own beliefs and continuity (hell I'm sure we all got out own, mine is a temporal cluster fuck similar to the original Universe story....), but it just drives me up the wall when people try to sell their beliefs and theories as facts.
It's "bad science", in other words sociobiology at its best (sorry to those Dawkins and E.O. Wilson fans out there, but if you think they are scientists I got some fun literature for you). I understand, there's a need to fill in blanks or try to discover the unknown, thought I don't know how you would do this without direct contacts at Hasbro/Takara, but in doing so it's best to not think of the most common denominator.