Changing a Users Role

Hello

Here is my situation, I was wondering if any of you have had a similar situation.

I have a user that needs a different role and profile depending on what time of the day it is. If it is regular working hours he should only be able to do regular DML through the application. But for after hours he is the on call person for the application so he must be able to do anything (i.e alter tables and procedures).

We don't want him to have this access during the day though. Is there anyway to set it up a trigger that checks the login timestamp to determine the time of day which will determine the role and profile he need's

I have looked into LOGON trigger's but they can not be used to change ROLE'S or PROFILES.

The version is Oracle 8.1.6 running on Sun Solaris 8

create or replace trigger log_on
after logon on database
begin
execute immediate 'set role lscx';
end;
/