When a child has been using the potty, but then they regress back to wanting a diaper, they usually have decided to hang on to that part of them self or they have become to busy to go potty.
Some children have trouble letting go of their poop because they are becoming aware of what is no longer theirs. They are still in the ego/mine stage, and need to hold on to something. By not pooping in the potty they feel like they are still in control. Once they relinquish this control, they learn that going to the potty makes them feel better.
Many parents tend to want to go back to letting the child use a diaper. I would suggest that you keep the child in underwear. Whenever the child has a potty accident have them help you clean up the accident. They need to understand what their body is doing, but they also need to take some responsibility and help clean up the accident.
Children like to feel responsible for their actions, and will generally get tired of cleaning up potty accidents. They will realize that it is much easier to just use the potty.
This will take lots of patience on the part of the parent, but in the long run will be in the best interest of the child. This will also allow time for you to teach your child proper hygiene.