The best switch I ever made was a little over 5 years ago when I went from my Babolat Pure Control + 'Swirly' to the Fischer M-Comp 95.
What was the immediate impact it had on my game? Well, anyone who's ever hit with that frame knows it is one of the least powerful frames out there. It allowed me to 'swing away' on most of my ground strokes, and gave me confidence to stay in ralleys, as opposed to forcing the issue, knowing it was just a matter of time before I made an error.
Why did I move away from it? Because Fischer sold their racquet company to Pacific, and they took too long to release the follow-up (the X Feel Pro 95). After 3 years, my frames were pretty beat up, I couldn't find grommets, nor could I find replacement pallets.
Now I'm using the Pacific X Force Pro and loving it, but the M-Comp 95 was the frame that introduced me to lower powered frames, and that's why it was the 'best switch ever'...
"Nobody beats Vitas Gerulaitis 17 times in a row" -- Vitas Gerulaitis, after beating Jimmy Connors on his 17th try.