Hi Tall,
Here is what I know. Go to the VI in the diagram that you want to replace-right click and selece the option "Replace" from the pop up menu. Then select the option "Select a VI" from the menu that pops up. Select the VI that you want to replace it with. You might have to save the vi with options as a liabrary in order to get the New VI in the liabrary.
See the picture attatched.
Hope it helps.
Good Luck.
-Pari.