Welcome! Once the crysallis is dry, and you can tell when it is totally formed and hardened after you have seen a few formed, you should just take off the top of the jar, put it on an empty jar while you clean the one it was in and then put it back. That is what I do. I don't like to move the cats once they make the J but once the crysalis is dry I can and do move the lids around, especially if other cats are in the same container. Pretty much about an hour after the crysallis is totally formed you can gently move the lid to another jar. I generally use the bigger 64 oz peanut butter jars (they are fatter than the smaller jars) to rear the cats in and other larger containers. I like to keep only 2-3 cats per jar but this year things have gotten a bit out of hand with all the eggs I found so early. I have been crazily moving cats around so they don't bother the ones that are J hanging. I have approx 20 crysallis right now, probably about 10 or so J hanging and about 80-90 cats of various sizes. I would love to take more in as there are so many more out there on my milk weeds and vulnerable to all the bugs on the plants but I simply can't take any more in. It is taking me a couple hours a day to feed the cats and clean the containers I have.
As far as the cat you think disappeared on your milkweeds, it probably went off to find a place to make its crysallis. They do not make them on the milkweed plants and sometimes travel quite far to find the right spot. I have only found two crysallis in the wild. I have looked alot all around the milkweed patches on other plants and trees near them but have only found two, one under our deck and one on the bottom of the siding on our house.