I have just called B&H to ask about switching my preorder from body only to kit. Not because I think I'll get it quicker but because since I placed the order I have thought more about what I already knew about my lenses... that is neither a 17-40mm or a 70-300L make good general lenses for a full frame camera.
Anyways back to ther reason for posting.
The guy at B&H said that
changing from body to kit will NOT change my relative place in line.
My order has been updated and it now has a new cost but the order number has remained the same. He says I should gte it early next week. As for if he meant they'll ship it then or that I'll actually get my sticky paws on it then... I didnt think to ask. I guess even if youre in the first batch presuming B&H arent taking extra staff just to ship this there will be a time required to fill all the orders. If Canon dumps a whole mountain of 5d3's on them they still need to be processed, packed and shipped.
Now I think about it logically (from the limited information I have

) your place in line is likely determined by your order number (or time)... so as they fill the orders they will move along a list of order numbers (or times) not have a separate list for each product. I am of course presuming the guy wasnt lying and they are told just to say yes and get more money out people when the option is there, you need to presume these things to function in like but you never can be sure can you...
So basically its not like lines of people at counters in a food court... you dont have to join the back of the noodle line if you decide you want to get noodles not pizza

You get to join the noodle line after the guy the joined the noodle line the same time you joined the pizza line...
DISCLAIMER: This is just an analogy... This is not how it works in the food court in the mall... trying to join the noodle line in the middle may result in you getting a punch in the face!
Of course depending on where your mall is results may vary... you may either just get frowned at or you may get stabbed... neighbourhoods can be really different in this respect.