More often than once you will encounter the issue that you need to change an init-parameter that cannot be changed during an active state.
SQL> alter system set sessions=500 scope=both;
alter system set sessions=500 scope=both
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified
Well - the beauty of RAC is of course that you can change init parameters in the spfile and bounce an instance. Your users won't notice - as the database is still available while you accomplish the change of the init parameter.
SQL> alter system set sessions=500 scope=spfile sid='ORCL4';
SQL> shutdown immediate
ORACLE instance started.
Total System Global Area 1862270976 bytes
Fixed Size 2072096 bytes
Variable Size 436208096 bytes