yes you can ...

SQL> create table my_table (my_column number primary key);

Table created.

SQL> desc my_table
Name Null? Type
----------------------------------------- -------- ----------------------

MY_COLUMN NOT NULL NUMBER

SQL> alter table my_table modify (my_column not null);

Table altered.
SQL> alter table my_table modify (my_column null);

Table altered.

SQL> alter table my_table modify (my_column null);
alter table my_table modify (my_column null)
*
ERROR at line 1:
ORA-01451: column to be modified to NULL cannot be modified to NULL

SQL> desc my_table
Name Null? Type
----------------------------------------- -------- -------------------

MY_COLUMN NOT NULL NUMBER


Note that it is the second modify to not null that fails, but not because o the PK definition ...

SQL> drop table my_table;

Table dropped.

SQL> create table my_table (my_column number null);

Table created.

SQL> alter table my_table modify (my_column null);
alter table my_table modify (my_column null)
*
ERROR at line 1:
ORA-01451: column to be modified to NULL cannot be modified to NULL