SQL & PL/SQL

Hi,
In the following simple sql query....all occurences of 'a' are replaced by the next character...

SQL> select regexp_replace('Palaio Faliro','[a]{1}') from dual;
 
REGEXP_REPLACE('PALAIOFALIRO',
------------------------------
Plio Fliro

Is it possible to get the opposite of this pattern... namely except for the 'a' character , all others to be replaced by their next character in the above string....
So , to get....aaa(i think... as only the 'a' character is not going to be replaced...)

Thanks...
Sim