Soul Echo — ruling 1

If you take damage between the start of the upkeep step and the time in which the upkeep trigger resolves, then Soul Echo will not affect that damage and you will be affected by the damage as normal.

Soul Echo — ruling 2

If you have more than one Soul Echo, the opponent chooses which ones will be replacing damage that turn (if any). And when damage happens, you decide which one of the ones the opponent selected that you want the damage to be replaced with, but the damage all goes to one of them.

Soul Echo — ruling 3

If this card runs out of counters before your next upkeep, the effect still replaces the damage and uselessly tries to remove counters from the now empty card.

Soul Echo — ruling 4

In a multi-player game, you choose a new opponent during each of your upkeeps.

Soul Echo : règles, fonctionnement et interactions (MTG)

Soul Echo is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This enchantment enters with X echo counters on it. You don't lose the game for having 0 or less life. At the beginning of your upkeep, sacrifice this enchantment if there are no echo counters on it. Otherwise, target opponent may choose that for each 1 damage that would be dealt to you until your next upkeep, you remove an echo counter from this enchantment instead.

How it works : Soul Echo

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
If you take damage between the start of the upkeep step and the time in which the upkeep trigger resolves, then Soul Echo will not affect that damage and you will be affected by the damage as normal.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
If you have more than one Soul Echo, the opponent chooses which ones will be replacing damage that turn (if any). And when damage happens, you decide which one of the ones the opponent selected that you want the damage to be replaced with, but the damage all goes to one of them.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
If this card runs out of counters before your next upkeep, the effect still replaces the damage and uselessly tries to remove counters from the now empty card.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
In a multi-player game, you choose a new opponent during each of your upkeeps.
No other card is named in this ruling.