Hi,
No u cannot change the parameter timed_statistics dynamically.In order to change the parameter enter it in init.ora parameter file and shutdown and bounce back the database.
This parameter can be changed dynamically in Oracle 8 and 8i.
In case of any help please be free to ask me at [email protected]

Regards,
Rohit Nirkhe,Oracle DBA,OCP 8i
[email protected]