Executing type cast functions in postgresql 9.1.3 is not the same as in postgresql 8.2.22. Concatenation is not working correctly

I was using postgresql version 8.2.22, then I upgraded to postgresql 9.1.3 and the upgrade was successfully completed.

But now some casts are not working the same as before!

In Postgres 8.2.22

I run these two queries and they both work correctly:

POSTGRES8222=# select TO_NUMBER('12345678',9999999999.99);

to_number
=========   
 12345678
(1 row)

POSTGRES8222=# select a ||'$'|| b from test;
 ?column?
----------
 1$abcdef
 2$ghijkl
 3$3456
(3 rows)

After upgrade to Postgres 9.1.3

I run the same queries and now they throw errors:

select TO_NUMBER('12345678',9999999999.99);

ERROR:  function to_number(unknown, numeric) does not exist
LINE 1: select TO_NUMBER('12345678',9999999999.99);
               ^
HINT:  No function matches the given name and argument types. You might 
need to add explicit type casts.

EXCEPTION
org.postgresql.util.PSQLException: ERROR: function to_number(numeric, numeric) 
does not exist

Hint: No function matches the given name and argument types. You might need 
to add explicit type casts.

Position: 150



select a ||'$'|| b from test;
ERROR:  operator is not unique: numeric || unknown
LINE 1: select a ||'$'|| b from test;
                 ^
HINT:  Could not choose a best candidate operator. You might need to 
add explicit type casts.

********** Error **********
ERROR: operator is not unique: numeric || unknown
SQL state: 42725
Hint: Could not choose a best candidate operator. You might need to  
add explicit type casts.
Character: 10

Why is it that casting in postgresql is not working as it did before?