The B-Theory holds that moments are ordered only by being before and after other moments in time. The A-Theory holds that moments also have temporal properties: pastness, presentness, and futurity. Note, then, that both A-Theory and B-Theory hold that moments are ordered identically, but A-Theory holds that there are temporal properties, i.e., that it makes a difference to the nature of a moment whether it is past, present, or future. Thus, a moment in time moves through time from being future, to being present, to being past. The problem is that for change to happen requires time. So, if A-Theory is true, then a moment in time changes from being future to being present. It does so in time, however, which it partly constitutes. This is a difficult topic to untangle, I think, due to A theorists being in a tangled theory.
Given the A-Theory, for a moment to be future, it must be future to us, now. The idea is that there is only one “now” which changes. It refers to the one moment which has the property of being present. That moment changes every moment, however. So, what happens when a moment loses the property of being future and gains that of being present? Well, just that. When does that happen? Just before that moment becomes now. When is that? What moment is that? There is no moment, except itself. This is the problem with the A-theory: it tries to allow for change in time itself. Time is the medium in which things change, however. This is related to the joke about time passing at the rate of 1 second/second. Time travel is inherently unit-less, since the units of time cancel out.
Okay, let’s back away from this for a moment and consider God. Some people believe that God was once outside of time, atemporal, and then became temporal. This means that he was once omni-present temporally, but no longer is. Thus, once he was at all times, but now he is not. How could this work? In what medium does this change take place? If it takes place in time, then God’s being in the future changes from being true to being false when he changes from being atemporal to being temporal. But he already was in the future. How can he change from being in the future to not being there? If we considered the future it as a location which someone could move in and out of, then it would be fine, but note that a person moves in and out of a room over time. There is nothing to distinguish the future where God is and the one where he is not: he once was in the future, but now is not. This might make sense if we had reached the future, but the premise is that it doesn’t matter when we are, God was still in the future once, but not anymore.
The same problem of change over time exists for moments as for God. When was the moment future? At any prior moment. When was it past? At any moment which comes after it. When was it present? At that very moment. Thus, from any moment, from its perspective it is present, which is what the B-theorist says, but the A-theorist says that moments change from having one temporal property to another. The problem is that moments, by their very nature, are the building blocks of time, and so a change to them is a change to time, or to the timeline. So, suppose there are four moments: A, B, C, and D. We begin at A. A is present, and the rest are future. Then we go on to B. A is now past, B is present, and C and D are both future. What changed? Did time change? Or did our position in time change? The B-theorist will claim the latter, but the A-theorist claims the former: that time changed. A change occurred to time. However, this change must occur timelessly. Since it is a change to time, it cannot also be a change which took place over the course of time. To refute this claim would be required to show that the A-theory is coherent. To prove this claim would prove that the A-theory is incoherent.
For time to change over time requires a time which time can change over. If time changes over a different time, then we will have to explain that time in the same way. If time changes over itself, on the other hand, then it faces the problem which an atemporal God becoming temporal faces: when was it what? That a moment was future, until itself, means that it either is always future--since that is the state of time--or never future--for the same reason. We could not notice a change in time over time, since the change would have to have occurred by that time. To note a change in time--that it is then then, and now now, and now was future then--is simply to notice that time exists, and is ordered in a certain way, completely consistent with the B-Theory--which is not to notice a change in anything.
Perhaps I will make another attempt later at making this comprehensible. As it stands, this post is hard for me to understand. The entire question of A-Theory vs. B-Theory, it seems to me, must rest on some kind of misunderstanding, but I do not know what that misunderstanding is.