…would she still be a Majesty? Or would she become a Highness? A Royal Highness? A nothing? Or what? (For that matter, would she still be a Queen, or just a Princess, or what?) Is she the Dutchess of anything? Would she lose that? Would she have to bow to Charles?
And what about her husband? What would he become? Would he still be a Prince? Would he lose all his Dukedoms and such?
The only instance of an English/British monarch abdicating other than under duress (Richard II, Edward II, Henry VI, Lady Jane Grey) is that of the present queen’s Uncle David, who was Edward VIII for eleven months. He remained a Royal Highness as a member of the royal house, though as you may recall there was bitter emnity between him and his brother an sister-in-law over his wife’s not being given equivalent stature. He was created Duke of Windsor, a new title (of which he was the only holder).
Philip, a member of a cadet line of the Danish and Greek royal house, is a prince in his own right, and was created Duke of Edinburgh in his own right. Elizabeth would be Duchess of Edinburgh as his consort at a minimum, and would no doubt receive some appropriate title from her son.
She is presently Duchess of Lancaster (and technically Duke [not Duchess] of Normandy in respect of her suzerainty over the Channel Islands), but those titles subsist in the Crown and would pass to Charles (as his Duchy of Cornwall passes to William).
She would curtsey to Charles, as her own mother did to her, because the monarch embodies the sovereignty of the United Kingdom. For Americans, think of it as something like the attitude expressed by the apocryphal Marine Gunnery Sergeant of the 1990s: “You are perfectly welcome to consider Bill Clinton a son of a bitch, but you will salute the President of the United States.”
However, short of a stroke or other incident that is not fatal but leaves her physically or mentally impaired beyond functioning (and perhaps not even then), the question must be regarded as purely hypothetical, as she has always made it clear it is her duty to serve for life, and that the royal family looks down on the “bicycle monarchs” of Europe who retire by abdicating.
UK practice wouldn’t necessarily be the same. But, for a contemporary example, Queen Juliana of the Netherlands reverted to the style of Princess Juliana when she abdicated in favour of her daughter Beatrix in 1980.
Queen Elizabeth died years ago.
Come now, we all know that Liberal was referring to HRH Elizabeth II.
That’s HM, please. 
Where’s the fun in admitting that? 
Wouldn’t she become Queen Mother? I know the typical usage is for the widow of a king, but it was my understanding it applied in the case of a Queen abdicating in favor of her heir apparent.
It is unprecedented, but she might be given the courtesy title of Queen Mother, given that she would be a former queen and she would be mother of the current monarch. That title has only been given to former queens consort, but a former queen regnant is unprecedented in English history (though not in Scottish history – Mary Queen of Scots was a former queen regnant during 1567-1587).
Yeah, but the now fawned-upon-for-tourism-purposes Mary Queen of Scots was chucked out of office because the Scots quite rightly couldn’t stand her, so it’s really not a parallel situation.
Gah! Please no. Jug-ears will be a disaster as King.
Could she get back in the line again? If Charles died, would it go back to her before William?
No - absent an Act of Parliament, abdication is for all time.
I suspect this would be the most likely scenario, party because she would indeed be the mother of the King, but also I suspect because the popular respect she has gained in the last 55 years would make it inappropriate to effectively demote her. A very different story from her uncle Edward.
But all her other titles that come as part of being Monarch would pass to Charles. And, of course, William would then be crowned Prince of Wales (and get all the other titles that Charles currently holds).
Even if she were of a mind to abdicate (which, for the reasons stated above, she isn’t) the lady has her eyes on the prize: 2015, during which year she would become the longest-reigning British monarch ever, beating Victoria’s 63 years and 7 months. Charles will have very brief reign indeed (that’s if he reigns at all).
And in 2012 we’ll have her Diamond Jubilee as well as the Olympics.
…or Brenda to her mates 
The moment she assents to the act of parliament confirming her abdication she’d become Her Royal Highness the Princess Elizabeth, Duchess of Edinburgh (HRH the Duchess of Edinburgh for short), which is what she was styled between her marriage to Phillip and her ascending the throne. Prince Phillip’s title wouldn’t change at all. Of course this is purely hypothetical; she will never abdicate. She’ll delegate more and more functions to Charles as she gets older, and might “retire” and have Charles made Prince Regent (as happened with George III & George IV), but she will die on the throne.
I tend to disagree.
What people need to remember (whether they love the royals or hate 'em) is that Queen Elizabeth II is a kickarse monarch. She’s dull and dowdy sure, but she’s very intelligent and, as former Australian PM Gough Whitlam described her (and if anybody should hate her, it’d be him), she’s the world’s most experienced living statesman. She’s been doing it for half a century or more, she doesn’t make embarrassing gaffes , she doesn’t dabble in politics, and she does the tireless round of boring stuff like school openings, and all without complaint.
The thing is, that’s unusual. British monarchs are supposed to be inbred fuckwits. It would be sad for us if we were to die without ever having seen one in operation. And ol’ Charlie might be fun. He’d put his foot in his mouth a lot, and he’d play up the pomp to ridiculous levels. And it’d be a relatively short reign before William takes over - and he’d be too modern and slick. I’m in no huge rush to see Charles take the throne, but when he does, it’ll be good for a laugh.
Cite?
Aren’t you forgetting George VI, Victoria, George III before his illness, Elizabeth I, Henry VIII, Henry VII, Edward III, Edward I, Henry III, and more?
Indeed, surely it’s remarkable how few idiots there have been?