I think what you're asking for is a scheme whereby some rows in a
table have datatype X in a particular column while other rows in
the very same physical table have datatype Y in the same column.
An alternative for NOT NULL columns would be to use a new attnum for the bigint version of the id, but add a column to pg_attribute allowing linking the new id col to the dropped old id col, to avoid the table rewrite.
Global read code change needed: on finding a NULL in a NOT NULL column, check for a link to a dropped old col and use that value instead if found. The check could be almost free in the normal case if there's already a check for unexpected NULL or tuple too short.
Then a metadata-only operation can create the new id col and drop and rename and link the old id col, and fix up fkeys etc for the attnum change.
Indexes are an issue. Require the in-advance creation of indexes like btree(id::bigint) mirroring every index involving id maybe ? Those could then be swapped in as part of the same metadata operation.
