As PaulRB says
Is it possible that I can get the results from all these sensors at the same time
Or I can get them only one after the other?
You can’t do more than one thing at the same time on an Arduino as there is only a single core in the processor.
Or is it possible to change the address of MLX90640?
Even if you could change the address you can only read them one at a time.
I think you misunderstood what multiplexing is all about. It’s full name is time division multiplexing, it gives a time slice to each device in turn. If this is done fast enough with the right sort of device, like an LED, it looks like they are all on at the same time due to your persistence of vision.