ORA-02095: specified initialization parameter cannot be modified

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You may get the following error message when attempting to execute a statement like:

ALTER SYSTEM SET DB_DOMAIN=yocoya.com SCOPE=SPFILE;

ORA-02095: specified initialization parameter cannot be modified

Certain initialization parameters are not modifiable using ALTER SYSTEM, so the only way to change their value is to perform the following steps:

1. CREATE PFILE='path to pfile' FROM SPFILE;
2. save a backup copy of this pfile in case of troubles!
   
3. edit the initialization parameter contained in the pfile;
   
4. restart the db using the newly created pfile instead of the default spfile: STARTUP PFILE='...';
5. CREATE SPFILE FROM PFILE='path to file';
6. restart the db normally, using the default spfile.

How do you if a parameter is modifiable?
If you look at the dynamic view V$PARAMETER, there is a column called ISSYS_MODIFIABLE, if the value of this column is FALSE, then the parameter is not modifiable using ALTER SYSTEM.
You can find a list of modifiable parameters in the Reference Manual (11g, 10gR2, 10gR1).

See message translations for ORA-02095 and search additional resources.

updated June 4: see Graeme's comment.