With reference to SQL DDL Manual. Chapter 1, Page 40: "The Teradata ALTER TABLE statement offers many extensions not offered by ANSI SQL. For example, changing column attributes in ANSI SQL is restricted to setting or dropping a default clause, which is not the case in Teradata SQL".
HELP SESSION; ....on my system shows me that the Transaction Semantics are 'Teradata'. Does this mean I am running in Teradata mode as opposed to ANSI mode?
I did think there was some type of allowance for altering char & varchar column sizes as long as you were increasing them. This would make sense as decreasing them would not protect the data.
You can change the datatype of a column as long as the physical storage remains the same, e.g. VarChar(5) -> Varchar(10), but not Varchar(10) -> Varchar(9) or Dec(5,0) -> Dec(9,0), nut not Dec(9,0) to Dec(12,0) or Dec(9,1)
And this is regardless of your session mode (although there are lots of differences in Teradata vs. ANSI mode)