It really depends.
Weasel is a retcon of my life and Mongrel is a retcon of his, as they're all alternate versions of myself.
Start by laying out the main events in the character's history.
E.G. Left by Mother at age 3, develops self-sufficiency but a fear of being alone.
Find out where the event happened that altered present history.
E.G. Mother returned at age 5.
Re-plot events after that time based on the loss/gain of self that that event caused.
Stase's "What if?" Story in Creative shows her alternate version; often the strings of fate do tend to pull you towards certain things.
A number of the "Elsewhere"/"What if?" Comics show very similar things (Iron Man being trapped in Medieval times, Cyber Batman fighting the Joker Virus)
The most important thing is to figure out WHERE fragments of the personality come from.
I.E. When Weasel was in Russia, as his emotions depend on the land he's in contact with, he was larger, more gnarled and almost completely naive. As he moved to America ("To Sleep, Perchance to Rest"), he became less violent(Contact with the Civil War/Red Indians), more flashy (MTV generation) and hyperactive.
Most characters are half personality and half experience, so if the experiences change, so does the character.
What Silver_Weasel said has a lot of sense. Events shape the characters.
Another angle is to consider how much has happened on-screen (harder to retcon) and how much has been only referred to (easier to retcon).
If your character had a fight with someone and ten people saw it and have been giving the opponent a hard time the past few months it's rather hard (I'd class it closer to impossible) to say it never happened or that it happened to with someone else. Hmm, of course the opponent may have been a changling or whatever.
Then again if your character has been complaining of having nightmares you can rather easily explain away that there never were any nightmares. They are unverifiable. Your character may have been lying about them. If you want to strike out any reference to nightmares ever you're left with the same sort of situation as with the fight example above. Still, your character may have been under the mental influence of someone who made her lie in the first olace, so it's not totally impossible.
The more you have to touch on-screen events the more clever explanations are needed, or a lot of suspension of disbelief.
My suggestion would also be to start building the character up from a scratch (chronologically) and see where that takes you.
Spider charts help, and so do understanding roleplaying partners. If they refuse to believe something didn't happen/happened another way, retconning becomes nigh-on impossible.
Assuming all those your character has interacted with understand you're retconning, and realise events may be wiped from their histories, you're set. As Coile said though, try not to change any on-screen event so drastically it makes the other characters re-write their characters.
Say your character in the original story, got pregnant...make a spider chart of everything that happened after that event that directly affected/was affected by it.
For instance, your character has an abortion, that would obviously have to be wiped if you were removing the pregnancy.
Marvel and DC retcon constantly, hence all the universes - the only difference there is they own the characters they're playing with, you only have yours.
Check with your roleplaying group/partner first, see if they have any conflicting hisories - and work from there.
Don't do what the Spidey/X-Men movies did and retcon into a very short time. Each event must have held significant impact, so you'll still go through quite long changes.
Picard does similar when Q shows him what would have happened if he gets all his childhood dreams. He beds his friend, stops a fight.....and ends up as an Ensign.
New events will sometimes take place though; If Bruce Wayne's parents are saved from the Joker, there will still be dangerous criminals running around Gotham, perhaps Dick Grayson will become the Batman? or worse....the Evil Batman.
OK, thanks for the input so far guys; it's helping me get a solid idea of how to go about things. Didn't actually consider what sort of impact on the emotional development a single change could have (if the character's lover had been killed by anything else, for example, she'd be considerably different). A silly thing, true, but I never seem to notice them until it's too late.
I have about six/seven hours where I'm just say on my [censored] travelling this evening (Holland, w00t, seeing Augury, w00ter), so I'll get started then.
Will let you know how things turn out.
Whilst you're with Augury, and you have spare time (
) try a look through Amber as I'm sure she has a copy. Each character has three differing versions, retconning based on the same experiences. Dora especially is a good start.
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I have about six/seven hours where I'm just say on my [censored] travelling this evening (Holland, w00t, seeing Augury, w00ter), so I'll get started then.
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OT I know but... I thought you two lived together? Wow... and I thought *I* had it harsh when I had to drive 80 miles to see my girlfriend (now wife)!
Stop worrying about silly games, go have fun with Augury and do the retcon when you come back!
@FloatingFatMan
Do not go gentle into that good night.
Rage, rage against the dying of the light.
Yeah; still apart, but we're getting better with things. Just really a case of establishing a few things, and then we're under the same roof permanently. 
Anyway, getting a sizeable chunk of the work done - being together doesn't mean we're down each other's throats all the time - so thanks again.
/EDIT - oops, just realised; this is Syra. Logged into Aug's system.
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/EDIT - oops, just realised; this is Syra. Logged into Aug's system.
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Don't worry, Me and Stase do that all the time
Simply put, how the hell do you do it?
While not strictly related to CoX, I'm reworking a great deal of my character's history in order to refine her past, sort out some conflicting background, things that contradict the lore and so on. Problem is, we're talking about well over a year's worth of real time characterisation and I haven't the foggiest clue where to go.
Anyone done this sort of thing before? If so, how did you go about it?
Right now the most likely option seems to be sitting down with a notepad and shifting things around that way, while talking over the changes that involve other player characters.
Any help with this would be massively appreciated.