Converting inherited partitions into declarative ones

Greetings.

I am looking into new partitioning of 10rc1 on a copy of a production
system.
And I'm having tough times with the full scan.

Per documentation:
> It is possible to avoid this scan by adding a valid CHECK constraint to
the table
> that would allow only the rows satisfying the desired partition
constraint before
> running this command. It will be determined using such a constraint that
the table
> need not be scanned to validate the partition constraint.

So I have this table with CHECK constraint:

test=# \d stats_201503
Table "public.stats_201503"
Column Type Collation
Nullable Default
---------------------------- --------------------------- --------- --------
------------------------------------------------
…
created_at timestamp without time zone
…
Check constraints:
"stats_201503_created_at_check" CHECK (created_at >= '2015-02-28
19:00:00'::timestamp without time zone AND created_at < '2015-03-31
20:00:00'::timestamp without time zone)

Still, if I try to attach it, I get Full Scan:

test=# ALTER TABLE jsm ATTACH PARTITION stats_201503 FOR VALUES FROM
('2015-02-28 19:00:00') TO ('2015-03-31 20:00:00');
ALTER TABLE
Time: 55502.875 ms (00:55.503)

Is it possible to avoid Full Scan here? I have TBs worth of data in
partitions,
so it'll takes ages to switch to the declarative partitioning the way
things stand now.

Thanks in advance.

--
Victor Yegorov