I had a feeling that's what you thought I said

I can't believe the idea didn't occur to me before. To use this method you have to have a "tank tool." OCD sells a tool/drip tip combo; I have a solid one.
1) Prime new carto, set aside
2) Place tank tool into the spent carto and push it about 1/3 of the way down into the tank.
3) Hold the bottom of the spent carto (so it doesnt pull back up) and pull the tool off.
4) Put the tool into the new carto
5) Turn the new carto upside down and push it into the tank until the tip goes into the old carto. Continue pushing until the spent
carto starts to be pushed out.
6) Turn the tank over so that the new carto is on the bottom and the spent one is coming out of the top.
7) Push the old one completely out and throw away.
8) Pull out tank tool, wipe down tank and insert a drip tip. (If you are using a tool/tip combo, rinse tool/tip and insert as a drip tip.)
9) Vape, vape, vape!
ETA: I did this for the fist time yesterday with a tank that was 2/3 full. The tank was a little wet on the top and bottom (like when you refill it), but I didn't lose any juice!