update old nodes vocabulary

hey there ..
I faced some issue with taxonomy..

1- create vocabulary named "sub books" and add some terms
-- term: D6
-- term: D7
2- In content type /books i add fields (Term Reference) assigned to "sub books".
3- Then i add some nodes.

After while i add another vocabulary named "main books"
-- terms: Drupal release
And i edit "sub books" term to be belong to "main books" term. With Reference field options module..
In the same content type /books I add another (Term Reference) assigned to "main books".
So When i add new node .. it display "main books" term and whats belong to the same term of "sub books"..

Parent (main books): Drupal release.
child (sub books): D7

My problem is:
When i edit the old nodes .. it's display
"main books" - none.
"sub books" - D7.

How i update the field "main books" of all old nodes to display like:
"main books" - Drupal release.
"sub books" - D7.

Hope you got it..