How does upsampling affect cd playback?

I was listening to the Music Hall Maverick SACD player the other day and discovered something that was startling. When playing a redbook cd, you can swith the sampling rates on the fly and there is a noticeable difference. With this particular disc, the soundstage seemed to move forward and there was more spaciousness to the sound. My question is why? How would sampling rates make a difference?

Thanks for your input.
The higher sampling rate allows the use of a less aggressive and higher-frequency analog filter. Less phase-shift results and thereby, the soundstageing is improved.
Thanks for the quick response. I asked that question at AR and was basically told I had imagined the difference in sound and couldn't trust my own ears! Your explaination makes sense.