NLS language questions

 I created a database using US7ASCII as character set.

 I tried to input some Chinese characters (8-bit double byte) . It  works! Even select statment!

 I would like to know to what happens if using a wrong character set.  And would like to know where I can get more info. about charcater set  in ORACLE.

 If I want to change the startup default character set, should I  re-build the database?

 Thanks in advance,

-- 
**********************************************************************
Kam-ming Ku (kmku)	          Big-5:[ÅUÀA©ú]   GB: ~{9K=uCw~}	
*********************************************************************