Official rule text
97.4 After a player gains a unit upgrade card, each of that player’s units that correspond to that upgrade card is treated as having the attributes and abilities printed on that upgrade card. Any previous attributes of that unit, such as the one printed on that player’s faction sheet, are ignored.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 97.4