I agree it makes sense to do it that way, and this is a very old thread, but technically doing a “save as” with a different sample rate at the output file format DOES do a sampling rate conversion in Wavelab. And it is sort of ‘behind the scenes’ so you don’t necessarily know, but In Wavelab 9 it uses the internal sox src with a setting of “best” to do the src, and In Wavelab 7 it used the internal Crystal Resampler with a setting of “high” (not “ultra”) to do the “save as” src. It’s not specifically telling you it’s doing a sample rate conversion, but it is.
But I totally agree with Arjan that it’s not a good idea to do a sampling rate conversion that way (as “save as”), for one thing because you might not have enough headroom in the audio to do an src without clipping, and there are other reasons. It’s much better to do it in a render with the src plugin with the options that the render allows.
The OP’s experience of not telling a difference even though the interface was set for 44.1, I find that it’s normal for the interface to switch sample rate if set for internal clock, and playing a file of a different rate from Wavelab. I believe that’s what the OP was experiencing, even though the interface probably said 44.1 before they started playing the 48k file, it most likely said 48k after starting to play the “saved as” 48k file.