This moving to the other side of the cashier or joining a newly opened one taking advantage of the fact others don't know or haven't noticed is an interesting debate, both sides have made some good arguments. I always think in life though that if you have to' justify' your argument repeatedly, that perhaps it's not as cut and dry as you'd perhaps like it to be? That's not always the case though and sometimes it's the other persons inability to understand a simple explanation or their unwillingness to change their stance. Sometimes though there's a right or wrong in both arguments depending on how you want to look at it.
Let's say we're queuing to pay our $50 for a plate of fries at Cosmic Rays with loads of other people waiting to be ripped off, angry in the knowledge that we're unlikely going to find a table after receiving our food and will probably end up eating it on a trash bin next to the restrooms stood up. We notice a rather bored looking lady wearing her very alluring CM outfit walking to an empty cash/order register and sitting down and realise she's very likely just about to open and began her most enjoyable of shifts waiting for people to decide what they want despite the fact they've had plain sight of a menu for the previous 15 minutes but have not bothered making their minds up. Now at some stage we know she's going to shout "This register is open now folks, come and fulfill my life by placing your orders and give me great job satisfaction" (or words to that effect)
Technically it's a new queue and yes, as such anyone can join it and those who don't notice or are slower to react can't really blame others behind them for being quicker to react and moving into this new line giving them a lesser wait than the person who was originally in front of them. Also it's not slowing down that person being served any as if that new register hadn't opened up, then their wait would have been the same as it's going to be anyway.
However if the world was perfect (and we know it's not), then morals and kindness would dictate that those nearby this newly opened queue would instinctively expect or tell those already in front of them to take that place as they entered the original line before you and as such should receive their food first. Now this 'argument' relies on 'kindness' towards others and not taking advantage of others hesitancy, ignorance, lack of confidence, lack of awareness or their own moral belief that joining that line instantly feels wrong to them as they feel that's pushing in. The fact of the matter is that you are 'taking advantage' of whatever makes those before you not move to this new line and get their food quicker, however you want to dress it up. You're technically doing nothing wrong but if you have to say that "you're technically doing nothing wrong" then this usually means at the very least you can understand to a degree how others may 'see' what you're doing as being 'not the kindest thing' and may feel slightly wronged over the situation?
Again it's not a perfect world though and we know that even if we were to say to others in front of us "Hey guys you can join that line now, go ahead you're before me", that many would look at us confused and start asking questions whilst other not so kind folk behind us would swarm to the new line anyway making our efforts probably worthless. The practicalities of doing this probably would cause confusion and it would take everyone in the world being kind and agreeing to be selfless to work, so it's probably highly unlikely going to happen anyway.
So no, technically you're not breaking any rules and I've done it myself for the reasons just given above. But let's not pretend we're doing nothing wrong, at least not in a perfect world anyway (which we know it's not)