Soul Tithe
Soul Tithe will refer to the original permanent.
Because its trigger condition includes an object, that object is the one that the trigger will see. It is determined when the ability triggers; the trigger doesn't care about the enchantment or what it enchants. “It” cannot be determined upon resolution, otherwise, the trigger wouldn't know when to trigger in the first place.
It would be different if the trigger was worded, e.g., “At the beginning of your upkeep, the owner of enchanted permanent sacrifices it unless he or she pays X, where X is its converted mana cost.” Here, the trigger doesn't attempt to determine what the enchanted permanent is until it resolves, so it would apply to the newly enchanted permanent.
There is no written rule for this exact case, but the spirit is very much the same as for 608.2i: if you care about the enchanted object for the cost or trigger condition, that object is determined when the ability is put onto the stack and locked in; if you care about it only in the effect, then you determine what the enchanted object is upon resolution.
Daniel Kitachewsky
L3, Paris, France
Rules NetRep