It's not easy to "change" an existing project to a different sample rate. However, if all you've done so far is import the audio files, not much is lost.
The thing is that Sonar (and in fact most audio programs) can only play back audio at one sample rate at a time. So if you want your project to be at 48KHz, your samples need to be 48KHz. Sonar tends to have a "flexible" sample rate until you drop the first audio file into the project or do some other audio related operation (like recording, or freezing a softsynth). Once the first audio file is in there at a certain sample rate, Sonar has no choice but to stay at that rate.
So probably the easiest way is to
1. Create a new project at your preferred sample rate
2. Create an audio track
3. Record some audio (really half a second is all it takes)
4. Drag in your BIAB samples with the different sample rate
5. Watch all your BIAB samples be converted to the correct rate