Postgresql copy data within the tree table

I have table with tree structure, columns are id, category, parent_id

Now I need a copy a node and its child's to a another node, while copying, the category must be same, but with new id and parent_id..

My input will be node to copy & destination node to copy

I have explained the tree structure in the image file..

i need a function to do so..,

PostgreSQL version 9.1.2

  Column   |  Type   |                    Modifiers                    
-----------+---------+-------------------------------------------------
 id        | integer | not null default nextval('t1_id_seq'::regclass)
 category  | text    | 
 parent_id | integer | 
Indexes:
    "t1_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "fk_t1_1" FOREIGN KEY (parent_id) REFERENCES t1(id)
Referenced by:
    TABLE "t1" CONSTRAINT "fk_t1_1" FOREIGN KEY (parent_id) REFERENCES t1(id)