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Modifying data is as transparent as adding new data. If you modify something, but do not change the parent_id field - the structure of your data will remain unchanged. For example:

// pseudo controller code
$this->Category->id = 5; // id of Extreme knitting
$this->Category->save(array('name' =>'Extreme fishing'));

1. // pseudo controller code
2. $this->Category->id = 5; // id of Extreme knitting
3. $this->Category->save(array('name' =>'Extreme fishing'));

The above code did not affect the parent_id field - even if the parent_id is included in the data that is passed to save if the value doesn't change, neither does the data structure. Therefore the tree of data would now look like:

• My Categories
  • Fun
    • Sport
      • Surfing
      • Extreme fishing Updated
      • Skating
    • Friends
      • Gerald
      • Gwendolyn
  • Work
    • Reports
      • Annual
      • Status
    • Trips
      • National
      • International
• Other People's Categories

Moving data around in your tree is also a simple affair. Let's say that Extreme fishing does not belong under Sport, but instead should be located under Other People's Categories. With the following code:

// pseudo controller code
$this->Category->id = 5; // id of Extreme fishing
$newParentId = $this->Category->field('id', array('name' => 'Other People\'s Categories'));
$this->Category->save(array('parent_id' => $newParentId)); 

1. // pseudo controller code
2. $this->Category->id = 5; // id of Extreme fishing
3. $newParentId = $this->Category->field('id', array('name' => 'Other People\'s Categories'));
4. $this->Category->save(array('parent_id' => $newParentId));

As would be expected the structure would be modified to:

• My Categories
  • Fun
    • Sport
      • Surfing
      • Skating
    • Friends
      • Gerald
      • Gwendolyn
  • Work
    • Reports
      • Annual
      • Status
    • Trips
      • National
      • International
• Other People's Categories
  • Extreme fishing Moved