You wrote:Now think of what this foretells: a baseej is cornered, switches sides and is carried aloft in the streets triumphantly.
Unfortunately, that doesn't say much to me. That says that a man was heavily intimidated, became afraid of what an angry crowd might do to him, and decided to momentarily join them. This man was part of a group that is murdering people in the streets. Do you think all it took to convert him was a nice conversation with a crowd? No way. If those images do in fact show a defection, they're only to cover his own ass in that moment.
Show me images of groups of Baseej putting down their guns as they approach a crowd. Show me someone unafraid for their life defecting. Show me Baseej fighting Baseej. That will be a defection. This? This is cowardice. Maybe I'm wrong (god willing, I am), but that's how it looks to me.
I'm one of the tweeters posting with the IranRiggedElect id. Considering the uniforms featured in your post, the forces are very likely not Basij. Basij usually don't wear professional uniforms like these. Amir Farsahd Ebrahimi, who is a former insider, gives a summary of different uniforms and corresponding forces here (in Farsi). If I'm not mistaken, Iran's official forces are a military, a police and an IRGC which has both a police branch and a military branch.
The basij are an additional unofficial force but the IRGC, faced with a need for manpower in the post-election unrest has repeatedly talked about plans to integrate the basij into the IRGC. In instances we have seen the basij wear vests to indicate their identity but most of the time they wear normal clothes with a religious touch and we call them "plainclothes". One particular day when the basij seemed to have been given vests was June 20, 2009 (see video here).
So my conclusion is that the forces that are seen in the December 27 video wearing anti-riot gear are NOT basij. They are either normal riot police or IRGC riot police. Why is it significant to make such a distinction?
Because the basij is a very ideological, unofficial, mostly voluntary group who may get paid on a case by case basis if necessary. They fight because they believe they are the soldiers of God and they are willing to sacrifice their lives. Whereas anti-riot forces, whether IRGC or not, are paid professionals who get a salary. They are less ideological than the basij. Much less ideological if they are with normal police rather than IRGC. They are the most likely to be looking for excuses not to hit people.
However, they are still very terrified of what would happen to them if they were to be caught on camera "converting" to green with uniform. I have seen the video of that incident and it is by no means showing a conversion. It is showing a desperate riot policeman trying to please the mob in a way that looks good enough for them to let him go free and yet "forced" enough for him to be able to tell his seniors "they forced me". He is aiming for this balance and not really joining the people. He might do that some day when he's not wearing his uniform. But with his uniform it would cost him his life.