just alter users's default tablespace to another tablespace you want, you dont have to drop the user unless that user has already objects created in tablespace system
you have all the commands in Oracle Docs
Yes, It is right that you don't have to drop the user but you can always alter user and change its characteristics.......but any time u create a user you have to have practice of defining atleast mandetory part of it.....it's like this,
Create user xyz identified by xyz
default tablespace abc
temporary tablespace temp..............