cast bytea to double precision[]

Hello,

I have a column of type bytea which contains a vector encoded in a 
string (elements are separated by spaces).
The problem is that it uses a lot of storage space.

I would like to convert this column to an ARRAY column with ALTER COLUMN 
TYPE.

If I understand correctly, I have to specify a function to cast bytea to 
double precision[]. Is it correct?

If this is correct, how to do that?

I use regexp_split_to_array to transform the bytea to a text[] but I 
can't figure out how to convert it to double precision[].

I'm using postgres 8.4.

Any idea?

Thanks in advance,
Mathieu