From man vncserver
vncserver is a Perl script which simplifies the process of starting an
Xvnc server. It runs Xvnc with appropriate options and starts some X
applications to be displayed in the VNC desktop.
tred@vaio:~$ which vncserver
So, open up /usr/bin/vncserver
with your favourite editor and search for Port
I see this at line 127:
$vncPort = 5900 + $displayNumber;
So, I expect that if you changed the 5900 to some other number, and restarted your vncserver it would use a different range of ports.
But why do you want to do this?
If you are thinking it will be "more secure" you are mistaken.
If you want vnc to be more secure, you should tunnel it through ssh