I've been doing some transplanting. And there is more to be done because I really want to sow again very soon, all the seed rests that are probably not viable anymore, some of my own seeds (Avonia!), and some conophytum seeds I bought this year.
So this is how it goes.
Step 1. Squeeze the container from all sides. This is why we use plastic and not clay ;)
Step 2. Pull out the plants. If they don't come out easily, stop pulling and squeeze the pot again.
Step 3. Now fill a pot with fresh dry pumice, up to the top.
Step 3.1. Examine your plants for bugs and remove all old soil from the roots. It's ok to pull off some roots as long as the main root is intact. Don't be squeamish. I usually rip off some of the main root as well if it's too long. If you are transplanting from dry soil into dry soil the plants won't mind (roots inactive).
Step 4. Use a stick to plant the seedlings into the container by dragging them down by the root (in case of adult plants you will need to wiggle the stick to let the pumice stones collapse around the plant and drag it down on their own). You can arrange them to your liking and put really a lot of plants all in one pot this way, saving precious space.
PS: I actually thought I could put a couple more trays on the windowsill but it looks like it's occupied now :D