Not quite bottom to top or visa versa. They are numbered sequentially from negative to positive. Start at the negative terminal of the sheet and follow the collector plate to the negative side of the cells. Go to the positive side of the cells on the other side of the sheet and follow the collector plate to the negative side of next brick group; rinse and repeat. They follow a sort of C shape around the sheet with a break in the middle with either a fuse or a bus bar.
Also, below is a picture for reference. This is an unfused sheet; you can tell by the bus bar below the coolant inlet/outlet.
Thinking aloud, if you pulled the sheets off, replaced the cells with newer ones, and replaced the cooling tubes from say a model S, then used nickel to remake the sheets, what would happen, I mean I know the 3.0 has the PEM upgraded but if the bms treats the cells as the lower capacity ones. would it just make it a new 2.0 pack. I mean 2400 cells can be had for as little as $1 each and complete model S packs are changing hands for $12k. I wonder what would happen if you had a bricked pack and swapped the cells out and added the water circulation from a model S and kept it in the same format.
Because the model S cooling tubes are easy to use, the nickel would be to replace the aluminium sheets forming the bricks. I know my welder will do nickel and cooper, so I thought I could make wide strips for the parallel connections and smaller ones for the series. It seemed to me using the removable model s coolant tubes would overcome the disaster of pulling the roadster pack apart, it would keep the same form factor and become a replaceable unit. But what would the PEM do, treat it as a stock unit?