Seedfork,
If a plant is already in your list, you have to use the "edit this item" feature in your plant list to make category changes. If you try to do it through the "add a plant" form, then nothing happens since the plant is already in your list. The system does make it appear that you have made a change, though, so a notice of "This plant is already in your list" would be a good addition.
In your plant list, use the highlighted icon:
