Orson has a hard time with this too.
If he's guarding the toy, i've taught him to "back away" so
i can grab it.
But if he has it in his mouth, about half the time "leave it" will work.
My problem with the trade out, is when playing fetch i dont know
which one he will chase. If i throw the alternative toy, he might still
want the old one or vice-versa... :? aggrevating cause i have to
go fetch myself the one he wouldnt chase!