Casting bytea to varchar

Hi:

I'm relatively new to postgres:

I've to alter a column which is of datatype bytea to varchar(255).

I used this:
ALTER TABLE tablename ALTER COLUMN columname TYPE varchar(255);

But I'm really not sure, if the value is casted properly.
Is there a way to explicity to cast the value, while altering a columns.

Example sql are welcome.

Thanks
Jayashree