Trazodone and Sertraline seem to be from the same class of antidepressant so maybe the GP thinks it would suit your mum best to change from one to the other. As the GP is the person who knows your mum their opinion would be worthy of a trial period. If you are concerned about the change I would just make a note of any changes you see in your mum, for better or worse, and then report these to the GP. I presume that as the meds have been changed that a follow up appointment has been made to check for this.