Alter column query has wrong syntax
`change_fields` is not part of the peewee migration API. Not sure what library you're using but I suggest taking the issue up there. For what it's worth, the playhouse migrate seems working well (it is not necessary or desirable to include the `primary_key=True` when converting field type - this is preserved):

```python
from playhouse.migrate import *
migrator = SchemaMigrator.from_database(db)

migrate(
    migrator.alter_column_type('users', 'user_id', BigIntegerField())
)
print(db.get_columns('users'))
```

Prints:

```
[ColumnMetadata(
  name='user_id', 
  data_type='bigint', 
  null=False, 
  primary_key=True, 
  table='users', 
  default="nextval('users_user_id_seq'::regclass)"),
 ...
]
```

SQL emitted:

```sql
ALTER TABLE "users" ALTER COLUMN "user_id" TYPE BIGINT
```
Hi everyone, I was running an update migration to alter a column type in PostgreSQL, but the query execution fails due to a syntax error.

Here is the migration file:
```python
def migrate(migrator, database, fake=False, **kwargs):
    """Write your migrations here."""

    migrator.change_fields('users', user_id=pw.BigIntegerField(primary_key=True))


def rollback(migrator, database, fake=False, **kwargs):
    """Write your rollback migrations here."""

    migrator.change_fields('users', user_id=pw.IntegerField(primary_key=True))
```

And this is the error message showing the query:
```
Creating migration "migration"
Migration has been created as "002_migration"
Starting migrations
Migrate "002_migration"
change_column ('users', 'user_id', <BigIntegerField: Users.user_id>)
Migration failed: 002_migration
Traceback (most recent call last):
  File "/Users/martin-zarza/Documents/My Projects/sendit-telegram-bot/venv/lib/python3.9/site-packages/peewee.py", line 3221, in execute_sql
    cursor.execute(sql, params or ())
psycopg2.errors.SyntaxError: syntax error at or near "BIGINT"
LINE 1: ALTER TABLE "users" ALTER COLUMN "user_id" BIGINT TYPE PRIMA...
                                                   ^
```

I believe there has to be a *SET* before the *BIGINT...*