Hi Antonio,
firstly , you can check this parameter dynamically switchable or not .
i have checked this parameters for you 
and it's a dynamic parameter, you dont need restart to activate this parameter.
oracle you can check v$parameter table for more information
SQL> select ISSYS_MODIFIABLE from v$parameter where name = 'optimizer_features_enable';
ISSYS_MOD
---------
IMMEDIATE it means that change takes effect immediately.
SQL> ALTER SYSTEM SET optimizer_features_enable='10.2.0.4' scope=both;
secondly, if you want to change by using pfile you can apply this steps,
1. create pfile from spfile;
2. pfile created in
for windows
Disk:\oracle\SID\11203\database
for Linux,Unix etc.
/oracle/SID/11203/dbs
after changed this parameter you must start with pfile
3. startup pfile=/oracle/SID/11203/dbs/initSID.ora
4. create spfile from pfile;
Regards,
Ümit Toptaş