Death knights are my favorite example of the changes 4.0.1. Death knights are getting one of the single biggest shocks in 4.0.1. The talent trees are being redefined so that blood is the primary tanking tree. Unholy and frost are primarily damage-dealing trees, with focuses on different playstyles. This is a huge change from Wrath of the Lich King, in which every tree could tank and every tree could damage. If you'd been a frost tank in Wrath, you cannot continue playing the same way you had been.
So what changed to institute this difference? What is our in-character justification? Obviously, you can use the same rationalization you've always used for respeccing a character (assuming you made that an IC dynamic in the first place.) The idea behind this IC explanation is that your character is in control of his powers and has chosen to use them in a different way. The failing here, of course, is finding an explanation for "why can't you go back."
Reach the next level
Of course, my favorite story explanation for why my character's powers have radically changed is that he has achieved a new level of mastery. This progression should be a little more complicated than simply screaming, "It's over 9,000!" Ideally, you've already been roleplaying your character's quest to expand his powerbase. If not, get started now.
You can travel to far reaches of the unexplored world, seeking out ancient tomes of knowledge and power. You could be exploring your inner magic in order to express your deepest reserves as new spells and abilities. You could simply have finally mastered the trick of getting energies to move just right and now let loose these abilities.
As to why you can't go back, that could be more complicated. Perhaps the old spells are now simply beneath someone of your capability. Perhaps it's an evolution or a permanent change, and you could no sooner go back to the old way of performing your magic than a butterfly could return to being a caterpillar.
Alternatively, your character's magical growth could be a sliding scale. Using the new abilities and new methods moves your scale further toward this next evolution. By the same rules, though, if your character accesses those old abilities, he would backslide. He'd lose access to the novel abilities and find himself bereft of those advances.
Going to the Light
Paladins are going through another big change. They will be eschewing the use of mana in favor of holy power. By using their other light-granted abilities, they build up holy power. When they build up a reserve of holy power, they unleash some other power, such as Templar's Verdict. They still have a mana stat, but it's no longer their sole resource.
Why the change? For our in-character purposes, the balance and gameplay reasons aren't as important as a story justification. For me, this seems pretty clear. The Light that grants paladins their abilities has chosen to more directly imbue their abilities into their chosen warriors.
I think of it as the paladin orders' choosing to become more pure. They've chosen to cast away the "dirty" methods of mana that have become so associated with lesser magic users. While priests and paladins clearly still use mana, the paladins are distancing themselves from mages and warlocks. Using Holy Power is a control over the mana, instead of the mana guiding them entirely.
Of course, if you decide to play the story such that the Light mandated this change, you could run a roleplay mystery for years. As your character struggles to understand and research the change in power source, you could explore the vast changes to Azeroth. Perhaps the key to discovering why the Light decided paladins can no longer use mana actually lies in the devastation created by Deathwing.
Fundamental changes to the universe
Deathwing's emergence back into the world and the vast changes he will cause can easily be expected to change the fundamental rules of the universe. We can assume that the laws of physics and magic will presumably change just as much as the physical terrain.
This isn't like someone set off a bomb under Stormwind. This is much more like an ancient god waking up and saying, "You know what? I don't like this teapot. I shall destroy it." The vast destruction and recreation is arguably the most immense and epic thing to ever happen to Azeroth. It is more than reasonable to think that everything will change. Hell, it shouldn't be shocking to an informed character to even find out sharp swords don't cut. A few spell and ability changes will be minor pittance.
All the World's a Stage is your source for roleplaying ideas, innovations and ironies. Let us help you imagine what it's like to sacrifice spells for the story, totally immerse yourself in your roleplaying or even RP on a non-RP realm!