Well, here's something to consider. A number of years after the establishment of the magical city of Dalaran (I
think -3000 DP), the people of the nation started running into problems with the concentrated magic essentially
tearing holes through the known universe into the
unknown, allowing demons and elementals to enter Azeroth willy-nilly. They eventually went to the Convocation of Silvermoon (the then-ruling body of Quel'Thalas), who were able to recite to them stories of quel'dorei's own sordid past -- specifically of the Sundering and the role that the kal'dorei played in that conflict.
As a result of what they learned, the Council of Tirisfal was formed to combat demons.
Now, in areas of strong arcane activity, it's really not difficult to bridge the planes and cause
something to appear. Summoning a demon may not be difficult, but I assume that controlling one would be -- unless maybe you reach a pact with the demon (a very common theme throughout history and a fun one to explore! I do so with my own sin'dorei warlock)
Regarding the transition from mage to warlock, I would think that it's very common for mages to become warlocks, more so than any other type of person; remember, fel energy is really a perversion of arcane energy. It wouldn't be surprising to find that some had undertaken studies in fel energy in the all encompassing pursuit of knowledge -- though it should be very evident that they did so in absolute secrecy since Dalaran in particular is very adamant in the destruction of demons and those who consort with them.
I've always imagined that, after the first few incidents regarding demons occurred within Dalaran, the populace reacted extremely to forcibly (and probably permanently) stop those who tried to consort with them. Perhaps due to the fear documented at the time, people who were accused of dealing with demons but lacked proof were treated the same way. That's always been the opinion I've held, but there's no direct evidence of it; conclusion drawn from suggestive data.
I understand that balance issues come into play that prevent people from being too powerful, but you can still allow your mage training to influence what you do and why. Ideally, your character is still working with the arcane, which is something that a mage probably worked with and studied quite a bit. However, that magic has been corrupted (typically by demonic blood). So, in theory, your character is still using the same magic that he possessed as a mage; it's just been corrupted and twisted into fel magic.
Specifically, magic associated with the Fire and Arcane trees would be the easiest to transition since the same types of magic is portrayed by warlocks; they have spells like Incinerate and Conflagration that are fire-based in nature, while we can always say that the arcane magic (which is much more malleable) transitioned into Shadow magic. Frost would be a little more difficult to explain, but perhaps the addition of fel magic would allow your character to explore its more negative traits: intense cold, absence of life, its killing ability, whatever!
Bottom line: you probably possess the same basic magic that you did as a mage. It's just been perverted by the fel, whether from you physically taking demon blood (I believe it's usually imbibed) or from consorting with demons. The fear of being too powerful is probably unwarranted since the magic is essentially the same; you have all of your old abilities but they've been twisted. They would be unable to exist in the same way that they had before.
Random aside, I found the quote in
this article to be a really interesting idea:
Quote:As demon cults continue to fester and grow in Desolace, Ashenvale Forest, and other remote corners, gruesome sacrifices are held to entice demons into giving their blood in trade. The cultists use their blood to enchant the weapons that they hope will bring about their masters' ultimate victory.
I think I'll be borrowing that idea for a character of mine!