I am bit disapointed especially because I was asking about this before buying printer and HP people told me it is possible to replace it in the middle of print.
Same here. I was actually surprised (and happy)when I was told that you could just pull the empty cart and add the new one, and continue with the print. I felt cheated when, after replacing a cart the printer cut and ejected my almost-finished print.
To compound the problem: Since HP has not included any way to tell the printer the length of a roll of paper, just imaging an ink cart running out toward the end of a 40x60 print. You change the cart only to run out of paper after about 50 inches of your roll on the 2nd attempt at printing the image!
I keep a running total of how much of each roll I've used. I also will remove a cart when I get the Very Low Ink message if I have a fairly large picture to print. I save the almost empty cart for when I have only a few small things to print so I can use up the ink without risking a big loss of ink and paper.
You can get a general idea after 30 or 40 square feet of printing just how much each color (on average) is used per square foot, so that when a given cart gets low enough to trigger the message you can roughly estimate how much you can get out of that color.