Yes. You can most definitely re-graft and have success. The rule of thumb is this, young seedlings have the best chance of 'taking' to younger growth of the Pereskiopsis. Of particular interest is 'active growth' if the stock is in active growth the success rate will rise. As the stock becomes more woody, as Navajoa suggests, you will find it very difficult to have a successful seedling graft.
Hope this helps a bit. Seedling grafts are simple in theory, however, they can become quite complicated and confusing.
If you want some more info I'm happy to help, I could blab on forever about grafts I do them all the time.
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