>> But the algorithm is wrong anyway - it does allow changing
>> NUMERIC(4,2) field to NUMERIC(4,3), which CAN corrupt the db.
>> (try it with value 99.99)
>
> So is my change from NUMERIC(18,4) to NUMERIC(15,2) safe?

Yes.
Both types use BIGINT as storage type, and the change will cause
the old values be divided by 100, so they will always fit.

Ivan