Swapping a new Actor or Object for one in already snapped shots

But of course, do the percolate swap function!

Simply load a shot which has the actor or object that is to be replaced
SAVE YOUR FILE. This is an important step because percolation functions aren't undoable. By saving the file now, if, for some reason, it doesn't produce the results you want, you can go back and reload your saved file.
Add in the actor or object which is replacing the existing actor or object.
Select them both without anything else selected
Go to the PERCOLATE menu
Choose SWAP OBJECTS IN SHOT
It will warn you that Percolate functions are not undoable. Press YES if have recently saved your file otherwise press NO. go back to step 2, do not pass GO, do not collect $200*.
Choose whatever range and other options you want in the menu that appears and click START PROCESS.
In some very rare cases, it won't be able to figure out which is the new object and which is the old one and will ask you to specify them explicitly, but 99% of the time it will just go ahead and do your swap.
When it does the swap, it will swap the position and orientation or any object, and additionally match the pose for actors.

* If you are a non-native English speaker, this is a reference to a game called Monopoly and all you need to know about is that you'll need to go back to step 2... :)