It is 100% possible to change someone's opinion in online debates, as long as it is not framed openly as a debate. Making someone feel like they're championing something against an aggressive opponent is not the way to go, it'll just make you come off as a hostile jerk from the other side. The instant a debate turns hostile, neither side will want to back down and the only result is a dumb internet fight of value only to onlookers.
The method of changing people's minds is calm persuasion, explanation, and an acknowledgement of the weaknesses of both sides of an argument at times. Use casual and familiar slang, be friendly. I have both changed minds and had my own mind changed, but have never done it while behaving in a hostile manner.
Oh, and cite your sources where you can, or whatever you say can easily be dismissed out of hand. Evidence helps.