Retroactively adding send and recv functions to a type?

Dear PostgreSQL General,

I wish to add the send and recv functions to a type I didn't write
myself. The help for ALTER TABLE seems to indicate this is not
supported directly. How can I achieve this?

The extension in question is hashtypes on pgxn,

https://pgxn.org/dist/hashtypes/

and I did not write it myself, but do want to add support for binary
send and recv.

I have a 48GB table already populated with sha1, and I'd rather not
have to drop it in order to upgrade the extension. Also, if I do
this, do I make the extension non-automatically-ugradeable? That is,
is there a way to write an upgrade script from a version without send
and recv, to a version with it?

--
Johann

I'm not from the internet, I just work there.