Ruin IV – Ruin IV currently replaces Ruin and Ruin III while under the effect of Further Ruin. With adjustments made to Ruin Mastery, Ruin IV will now replace Ruin II.

The bolded means that they are changing or altering the function of something. Notice how replace is used twice. The current function is explained and the new function is explain in relation to the previous explanation.

Ruin Mastery – Ruin spells will be seeing some adjustments, so in accordance to this Ruin Mastery has been changed so that instead of Ruin and Ruin III being upgraded to Ruin IV, Ruin will be upgraded to Ruin III.
A new trait which will upgrade Ruin II to Ruin IV will be added.

The Ruin IV explanation above is kinda redundant information from them. We already know Ruin 1 and Ruin 3 upgrade to Ruin IV on proc. This is changing. So instead, Ruin Mastery is being retooled to deleting Ruin I and a new trait trait will control the function of Further Ruin.

Reddit is arguing over a rushed community translation.