>I checked char fields & none has any '~' tilde in it.
>checked NULLDISPLAY as well.
>Still same ERROR, or bulkInsert. String too long error is shown.
>
>If I allow nulls for all char fields then the upsizing is successful.
>allow nulls for any 1 or few char fields is not successful, only selecting all char fields avoids the error.

You may be able to do a 'trace-back' based on the data uploaded to SQL Server by enforcing the NOT NULL rule on the fields one at a time. This will hopefully, point you to the field or fields that would have failed during the upsizing as SQL server will not allow the table changes if it breaks the constraint.