If I have a Platinum Emperion on the battlefield and I tap my Tree of Redemption, will the Tree still change it's toughness to my current life total, or will the Platinum Emperion negate that effect, since my life total is unable to change?
The Tree of Redemption will not be changed.
701.8. Exchange 701.8a A spell or ability may instruct players to exchange something (for example, life totals or control of two permanents) as part of its resolution. When such a spell or ability resolves, if the entire exchange can’t be completed, no part of the exchange occurs.
Also see this ruling on Platinum Emperion:
If an effect would cause you to exchange life totals with another player, the exchange won't happen. Neither player's life total changes.