Thank you for your posts everybody. Apology accepted Mudgel, and I apologise too for whatever part I played in causing confusion.
 

I think I get it now, but just to make sure:
 
There are 3 areas of bit depth in question: the first is the bit depth of the clip(s) which, in this example, is 16 bit; the 2^nd is the project bit depth (as displayed in Sonar’s tool bar just below the sample rate) which in this case is 24 bit; and thirdly there is the bit depth option at the export stage. Let’s say hypothetically that there was no bit depth option at the export stage and that the bit depth of the export remained exactly as is. The bit depth of the resulting exported audio would reflect the bit depth of the clip (16 bit) and not the bit depth of the project (24 bit), right?

Yes.

In Preferences/files/Audio Data make sure that for Import Audio, Original is selected. This is Sonars default

With the above setting at default (Original), any imported clip will retain its original bit depth on export.

If you want however you can change this behavior and select 16, 24, 32, 64 for bit depth, perhaps in line with your project bit depth.

This is a simple job for any wave editor. 
OPEN, Copy, Paste, SAVE.