If you control but don’t own Old-Growth Troll, you (not its owner) will return it to the battlefield when it dies. You’ll choose which Forest you control it will enchant as it returns to the battlefield.

If there’s nothing for Old-Growth Troll to legally enchant, it remains in its owner’s graveyard.

If a token is a copy of Old-Growth Troll, it won’t return from its owner’s graveyard.

If a nontoken permanent that's a copy of Old-Growth Troll dies while it is a creature, it will return to the battlefield as an Aura with both its own normal abilities and the two new abilities granted by old-Growth Troll. The resulting permanent will also retain its name, colors, and supertypes, but it will be an Aura enchantment with no other types or subtypes.