So, is the "new" ancestor now the direct ancestor of the "original" ancestor?
For example before you had: w_concrete from w_master and w_master from window
and now you have: w_concrete from w_master, w_master from w_base and w_base from window?
If so, did you do this by creating a new window and save as w_base, then edit source of w_master and replaced from window with from w_base, twice? savedok?