UMLR — How to deal with guard in Transition redefinition?

In p. 336 (i.e., 14.3.3.1.2 Transition redefinition) of UML 2.5 specification, the spec. says "A Transition of an extended StateMachine may in the StateMachine extension be redefined. Transitions can have their effect and target State replaced, while the source State and trigger are preserved." It does not mention the guard property of Transition, so, the guard property must be preserved or can be replaced? Any ideas?