Each Patronus is unique

[Inge]

No: HPFGUIDX 163864

According to JK's answer as to how Order Members communicate with each 
other (Section FAQ in her own site), each Patronus is unique.
What she says is (quote):

"The Patronus is an immensely efficient messenger for several reasons: 
it is an anti-Dark Arts device, which makes it highly resilient to 
interference from Dark wizards; it is not hindered by physical 
barriers; each Patronus is unique and distinctive, so that there is 
never any doubt which Order member has sent it; nobody else can conjure 
another person's Patronus, so there is no danger of false messages 
being passed between Order members; nothing conspicuous needs to be 
carried by the Order member to create a Patronus."

Yet Tonks' Patronus *does* change - and if one can change, then 
probably other Patronuses can change, too. So how does that fit with 
Rowling's wording "there is never any doubt which Order Member has sent 
it" ?
Just wondering (again),

Inge