“Soul Exchange” targets. Select the target at activation.
“Soul Exchange” allows you to Tribute the targeted monster as though it was yours for the rest of the turn. After resolving “Soul Exchange” the next action you make that requires a Tribute must use the targeted monster.
You can Tribute the targeted monster for any card that requires a Tribute, or for a Tribute Summon. So you could Tribute it for the effect of “Cannon Soldier”, a Ritual Summon, or for the cost of “Monster Gate”.
“Soul Exchange” does NOT give you control of your opponent’s monster, so you can use “Soul Exchange” combined with your “Cannon Soldier’s” effect even if you already have 5 monsters in your Monster Card Zone.
You can activate 2 “Soul Exchanges” and target 2 different monsters. You could Tribute them both for the Tribute Summon of a Level 7 or higher monster.
You cannot activate “Soul Exchange” during your Main Phase 2 because you must skip your Battle Phase.
You can target a face-down monster, but you cannot Tribute it for a Ritual Summon or for “Metamorphosis” because you do not know its Level.
You cannot use “Soul Exchange” for “Polymerization” because “Polymerization” does not use Tribute.