What to watch out for when ALTERing NUMERIC(38,0) to BIGINT?

AWS RDS Postgresql 12.10

We've got tables with columns of data type NUMERIC(38,0) which are a legacy
from an Oracle migration.

Besides what's mentioned in
https://www.postgresql.org/docs/12/ddl-alter.html#id-1.5.4.8.10, what
happens *internally* when I run:
ALTER TABLE foo ALTER COLUMN bar TYPE BIGINT;

I'm thinking mostly of record fragmentation.

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