If you have already purchased a node on the Skill Tree, you will have full access to the bonuses conferred by that Node. However, NEW nodes that you want to purchase may require a Key to unlock, depending on where they are on the tree.
Only the top two "tiers" of nodes on the Skill Tree will require a key to unlock them.
They do not replace Skill Points. You will still need to spend Skill Points to purchase specific nodes, but some nodes will require that you unlock their "tier" via a Key first.
I'll share some images along with the longer update to help communicate this visually.
Just to be clear though, no existing Skill Tree progress will be lost!
For what its worth, there will be a Bronze, Silver and Gold Eliza Prize Fight event that will allow you to compete with other players of your skill level for different levels of rewards.
The Welcome Pack (and, by association, Valentine - Oh Mai) has been disabled for now and will not be included at launch. However, we are planning to have other opportunities to get her in the future!