Changing partner happens more often than you might think.
As a PM on 3 ERP implementations, I have had to manage changing partner on 2 of the 3. The third implementation was direct with a vendor so changing partner was not possible anyway.
On the third implementation we had to fire our vendor project manager...twice.
Why did we have to change partner?
a poor partner selection decision became clear once we implemented
partner performance was not considered to be acceptable by the steering team
Changing partners is both time-consuming and costly, so no project wants to do this unless it has to.
It's time consuming because you have find a new partner, evalauate them and rebuild the partner relationship. Then you may have to change the implementation process to fit with how the new partner does it. This all causes project 'turbulence' especially for the delivery team.
It's costly because you have repeat parts of the project to get the new partner up to speed on your solution requirements and processes. And their statement of work may turn out to be more expensive than that of your original partner. Steering team approval for the budget changes will be needed.