You can activate “Soul Exchange” during your Main Phase and use its effect to Tribute your opponent’s monster for “Ectoplasmer” during the End Phase.
If you activated “Soul Exchange” targeting your opponent’s monster, if the monster is face-down, you cannot Tribute it as a cost for “Metamorphosis”.
Since “Soul Exchange” targets, “Spirit Reaper” is destroyed by its own effect before you can Tribute it.
Netrep Rulings:
The player activating “Soul Exchange” must declare a target at the time “Soul Exchange” is activated.
If the monster targeted by “Soul Exchange” leaves the field before it is Tributed, that effect of “Soul Exchange” disappears. “Soul Exchange” will still force the player to skip his or her Battle Phase.
If the effect of “Soul Exchange” is not used in the turn that “Soul Exchange” has been activated, the effect of “Soul Exchange” disappears.
Monsters targeted by “Soul Exchange” may be used for a Ritual Summon.
If “Soul Exchange” targets “Spirit Reaper,” “Spirit Reaper” will destroy itself by its effect.
Netrep Q&As:
Q: When is the monster targeted by “Soul Exchange” Tributed?
A: The monster targeted by “Soul Exchange” remains on the field until you actually use it for the Tribute. The monster is not automatically Tributed when “Soul Exchange” resolves. Curtis Schultz, 02/12/2004