The ALTER COLUMN ... TYPE command should be implemented in the declarative schema changer. This issue is used to track the work for trivial changes. Trivial changes are data type changes that don't require a backfill or a shadow column. Only update to the type in the descriptor is needed. One example of a trivial change is to alter the type from a SMALLINT to a BIGINT, or CHAR(10) to a CHAR(20).

We should make sure to avoid as many of the known limitations tracked in CRDB-25314 as we can.

Epic: CRDB-25314

Jira issue: CRDB-39782