I know the manual clearly states in "Default Settings for New Projects":
 
"After audio has been added to the project—either by recording audio or by using the File > Import > Audio command—you can no longer change the sampling rate for that project."
 
But I have a large project and only now I realized that it should be at 48000 Khz and not 44100 (default).
Is there any way to change this?

SONAR has great SRC on imports but once audio exists in a project, the sample rate gets locked. The only way I know of is to export each audio track individually (batch export as broadcast waves), close that project, create a new project and set 48K, then import those tracks (select them all in the dialog).

There may be a simpler solution, but not sure.

Use a 3rd-party conversion utility like R8Brain Free to batch-convert all existing audio files in the per-project audio folder with the project closed. R8Brain works particularly well for this because it will rename the original files, leaving the new ones with the original names. When you re-open the project in SONAR, it will set the project sample rate based on the audio files it finds, and the conversion of the project should be totally automatic and transparent with no sync errors or other weirdness.

Not an issue for me, but cool solution!

+1 That one is very good to know. (I thought the sample rate was cwp-dependent)

I tried the r8brain solution (44.1 to 48 ) and it worked very well. The only problem was that SONAR recognized the audio clips as shorter than the original ones so I had to re import the resampled ones by r8brain to get them complete (not a big deal in my case).

If the issue is delivering the project at 48 kHz, why not finish the project, mix it, then sample rate convert the master to 48 kHz?