Fixes what the asset is (a mistyped grade, a wrong description), as distinct from which loan it backs (withdraw). Partial: keys present are replaced, keys absent are untouched, an explicit null removes one. Correctable keys are those the asset type declares in table_columns. Identity and asset type are immutable. Requires an ACTIVE pledge on this loan. The replaced values come back in changes - the only record of what was overwritten. Requires CORRECT_COLLATERAL_ASSET.