I actually discussed with my wife after I finished ME3 how Bioware will approach this (given that it seemed inevitable that the ending would be altered - for the record I like the ending as-is), and my first thought was "oh, well they wouldn't be so brash as to sell us the "real" ending, so it'll be a title update".
My wife (who hates the ending and is a big fan of various other theories around the interwebs) said "Nope, if it's a title update it'll be compulsory otherwise you get knocked offline."
So free DLC? Basically like a title update but optional. This is the best hope, I guess. But it's kind of academic, why not make it compulsory. Ok, so maybe PC gamers have it a little rough here since Origin and Steam mean you only get a digital version of the game which is permanently modified. If you like the ending, you have no real control except to never update the game. At least my disc version, now pressed, can never be amended. No matter what DLC I buy or title updates are pressed on me, all I have to do is clear the cache and refuse to apply the "patch" and I can see the original ending.
Here's a film example dear to my and my wife's hearts. The Rocky Horror Picture Show. My wife grew up watching the movie. Had it recorded on VHS, bought the VHS, watched it over and over. She knows it inside and out.
In recent years, for it's 25th anniversary, it was remastered. Suddenly, when we decided to buy it on DVD, all we had was the remastered version. Unfortunately the remastered version has the music from the soundtrack dubbed over the original recording. It does sound crisper, but it's absolutely not the same thing, not to someone who has watched the movie dozens of times since a young age.
It was watchable, but annoying. So... we turned to Bitorrent. The original 1976 version in it's undoctored glory. We had to break the law just to receive a technically inferior product to the version we already owned legitimately.
There are ways around this kind of interference by the creator in every form of media. Gaming allows the publisher to be more invasive, sure, but I guess that's a price we had to pay in exchange for convenience. Some films were recently removed from Netflix. Anyone who eschewed DVD in favour of the cloud... sorry. Granted, those films are probably a buck apiece in some bargain bin somewhere, but it sets an unfortunate precedent.