ORIGINAL: proptop
That change of trim when you flip your triple rate switch could quite possably be in your transmitter (? ) not the plane...
I encountered the same thing years ago with a Hitec Flash 5 Tx. so I sent the Tx. back and they "recalibrated" it...
Try a test while on the ground...watch for change in elevator trim as you flip the switch.
Oh, I checked the obvious stuff 10 seconds after I landed it when the change in pitch was noticed. It's not in the radio set-up. It's on a Hitec Aurora 9 that I fly everything from my 35% Extra 330 to my smallest heli.