...we wanted to upgrade a QM's VM, as we had HA QM's, I explained the admin they could use WMQ' switching mecanism to upgrade VMs without stoping the service...

Anyway the command is :

endmqm -s <QM NAME>

Which makes the active QM switch to the standby instance.

Ok but when I tried to make the switch, I was getting this freaky message AMQ7276 : WebSphere [...].

I was searching a answer to this on google, then I realized the solution was kind obvious :P

...yes, when you wanna switchover, you need to have the standby QM in standby mode, start it with this command : \o/

strmqm -x <QM NAME>