What class/method responsible for copying PO change version into active version

Hi all,

         I would like to know the class/method that is responsible for copying changes into active version.

1. Create PO (saved version).

2. Order PO (Work flow triggered i guess).

3. Work item approved. I am guessing this converts changed version into active version(all changes are copied to active version. Change version is closed). Which class/method does this?

Thanks,

Eswar.