pentimentosound
 No one mentioned whether downward conversion causes artifacts.... yes, no?

Downsampling is unlikely to cause artifacts if it is properly implemented, but the downsampled file must necessarily have less information than the original. If the original file sampling rate is near the Nyquist rate (required to accurately reproduce the original signal) then downsampling to a lower rate may bring you into a range where the original signal is no longer accurately reproducible. The loss of samples will produce a file that can not reproduce the original, and if you upsample that file, it will be impossible to recover that fidelity because the critical information was lost in the process. Upsampling does not produce any new information. Generally 44.1 K will accurately reproduce the audible range of audio, so that is the lower practical limit that you should downsample to. 

Don't forget about the new SONAR render-with-upsampling option. I've been experimenting with it for a while, and with some mixes using some plug-ins, it definitely adds clarity to 44.1 kHz projects that's not obtainable with 44.1 kHz alone.

Try Voxengo R8brain ... its free and IIRC it has a batch mode.
You should be able to change your project to the new SR, close it, then convert the wav files and reopen the project.
I think that is the procedure I used
Last time I used it was way back when I   was fooling around with 96Kb. Could have been with 8.5 or X1.
It will definitely convert waves to whatever bitrate you want.
 

Didn't get to catch all of this thread, but Beeps' points are good ones. As I have only done a handful of videos, it is also simpler to work to completion (if in 44.1K) and then convert the master to 48K when the time comes for video work.

I get the doubling thing, from having read (retained? maybe?)Craig's posts on FX at 96khz etc. and "the new SONAR render-with-upsampling option", and that was the other reason for going to 48khz; to try  going to 96...
Since my 18i20/OctoPre Mkii can both do 96( or 88.2 but that seems useless with the desire to use tracks for video etc), and I have better mics/pres than the last time I tried 48khz, I'd like to "move up in the world", so to speak.
LOL
Michael