As an example: Upgrading a pair of A7X to a 5.1…
Use A7V Series at front of setup.
They are way more capable than A7X, and most material in 5.1 sits in the LCR channels (left, centre, right). The less capable A7Xs can be used at the rear for lighter duties.
The front loudspeakers should be set to UNR for tonal compatibility.
There can be issues with the different phase responses of different loudspeakers so ideally one would use exactly the same model in a 5.1/7.1/3D system.
The first compromise to this is using smaller loudspeakers for the rear and height loudspeakers.
A second and less desirable compromise is to use a different series of loudspeaker for rear and height loudspeakers. It would be very unusual for a customer to mix different brands in a single system.
In all cases the left/right pairs in a system should be the same model.
Finally, all DSP loudspeakers have latency whereas analogue loudspeakers do not.
To overcome this issue in mixed analogue/DSP systems the analogue loudspeakers should be delayed 1.7 ms or placed approx. 60 cm (2') further away from the listening position.