OK, I read your first post wrong.
If you need to change to another user there are a lot of places that you need to configure, like mysql database, FreePBX etc.
This is from /etc/init.d/asterisk:
# Uncomment the following and set them to the user/groups that you
# want to run Asterisk as. NOTE: this requires substantial work to
# be sure that Asterisk's environment has permission to write the
# files required for its operation, including logs, its comm
# socket, the asterisk database, etc.
#AST_USER="asterisk"
#AST_GROUP="asterisk"