Don't know if you can do this with the trees in the pots. Normally you would turn the pots up side down, wet the pots so you have a damp surface and add an acrylic fortifier to your mix. After the bottom and sides have setup you turn the pot over and finish the return on the inside of the pot. You only have to cover down to where the soil would start inside the pot. Since your pot is starting to deteriorate I don't now if it can stand up to the removing of the trees or any of the other process.

Thanks for the info on the acrylic fortifier. I'll be sure to add that in. I definitely think it is worth experimenting with these pots to see if I can reuse them for other plants. They just have a thin layer on the outside that is flaking away at this point so they are still strong enough to work with. This may be one of those projects that sits in the garage in the "planning" stage for a while before I get around to doing it but it sounds like it can be done.