Increasing size of array items without locking

Hi. I'm trying to increase the size of items in an array column on a large production database but it's locking the table. Is it correct than when you make a field larger, it shouldn't lock because it doesn't have to check the constraint on every row? For example expanding a VARCHAR column from 255 chars to 2048 chars.

Is the problem here that it is an array field?

ALTER TABLE "msgs_msg" ALTER COLUMN "attachments" TYPE varchar(2048)[] USING "attachments"::varchar(2048)[];

Thanks

-Rowan