What are the effects of up-sampling

My last CDP purchanse had an option to select non-upsampling or 24/96 upsampling. After listening to some of my new(er) high quality recordings I concluded that up-sampling had no benefits for me. The sound was more closed in, i.e. rolled highs, etc and I listening in non-up sampling. Recently I put on some old CD's, either early digital or re-master analogue, which erred on the bright/glary side of neutral, in upsampling made and they became much more listenable.

Is this a common experience with up-sampling? Is it perhaps just connected to my tuning of my system? Or is it just the way my CDP, a Raysonic 128, is configured? Or, am I just hearing things, or not?
Hello Newbee,
You are not hearing things.I also switch between modes and I agree, early CDs benefit most. I have the choice between dCs and Zanden. The latter does not upsample and generally sounds best, only to be beaten by CDs ripped to my laptop and then played through Steve Nugent's "Spoiler" UBS DAC.
Detlof, thanks for your validation AND patience in reading thru my poorly constructed and unedited(!) post. :-)
I agree with you as well. I had a dCS Purcell for a number of years, and experienced the same effect through my Audio Logic DAC. Ultimately, the upsampling became unnecessary as I played more and more recent recordings, and the added noise from the upsampler became a negative.
The technical effect is a smoother waveform.

A smoother waveform may or may not sound better...it depends on many factors.