making sequence bigint

oracle doesn't know "bigint". it knows NUMBER data type, and FLOAT ... for large numbers.
which is defined in the target column, not the sequence object
http://www.techonthenet.com/oracle/sequences.php
with MAXVALUE 999999999999999999999999999 being the default, that should be big enough?

so you might want to clarify the question or actual problem?

what will be max value for below sequence?

CREATE SEQUENCE  SEQ_D_DATE
START WITH     1
INCREMENT BY   1
NOCACHE
NOCYCLE;