Since here on LW changing one's opinion is considered a supreme virtue, I would even suspect that the long-term users are confabulating that they have changed their opinion when actually they didn't. Anyway, a technique that might be useful is keeping detailed diaries of what one thinks and review them after few years (or, for that matter, look at what one has written on the internet few years ago). The downside is, of course, that writing beliefs down may make their holders even more entrenched.