Well to be honest, if I have such a change of length in a string field in CMOD, I change simply the size of the string in the database (table arsagfld), close the current segment table, and voila.
As long as IBM doesn't change that table layout. There are no problems, if you only increase the size.
Of course if you want to change old entries in the old segment tables, then... you need to play with alter tables, and so on.
In all cases, Testing Testing is your friend, as this method is totally unofficial :-) and not supported! But works like a charm!!!
Regards,
Alessandro